decolint

decolint is a linter for Dev Container configuration files. The following file types are supported:

• Dev Container definition (devcontainer.json)
• Feature (devcontainer-feature.json)
• Template (devcontainer-template.json)

It checks for common mistakes, security issues, and best practices in these files. See Rules for the list of checks decolint performs.

A container's final configuration comes not only from its own file but also from the base image and the Features it uses. With -merge, decolint resolves all of these as they would be at runtime and lints the fully merged configuration.

Installation

decolint can be installed as a prebuilt binary, as a container image, or from source with Go.

Prebuilt binary (Recommended)

Download a prebuilt binary from the releases page. Release artifacts are signed and carry build provenance; see Verifying release artifacts.

Docker

docker run --rm -v "$PWD:/workspace" ghcr.io/bare-devcontainer/decolint [directory ...]

Images are published for linux/amd64 and linux/arm64 and tagged latest, <major>, <major>.<minor>, and <major>.<minor>.<patch>. They carry the same build provenance attestation as the binaries; see Verifying release artifacts.

Install with Go

GOEXPERIMENT=jsonv2 go install github.com/bare-devcontainer/decolint/cmd/decolint@latest

GOEXPERIMENT=jsonv2 is required because decolint uses the still experimental encoding/json/v2 standard library package.

Usage

decolint [directory ...]

Each directory is detected as one of the following based on its layout, and the configuration files it contains are linted:

• Dev container definition — the devcontainer.json files at the locations defined by the devcontainer specification: .devcontainer/devcontainer.json, .devcontainer.json, and .devcontainer/<folder>/devcontainer.json
• Feature (contains devcontainer-feature.json) — that file
• Template (contains devcontainer-template.json) — that file, plus the dev container configuration the template ships

With no arguments, the current directory is linted.

Configuration

Every linting setting can be declared in the config file. The four scalar settings additionally have a command-line flag, which overrides the config file when given:

Setting	Config member	Flag
Target platforms	platforms	-platform
Merge features	merge	-merge
Deny warnings	denyWarnings	-deny-warnings
Output format	format	-format
Category severities	categories	—
Rule severities	rules	—

-merge and -deny-warnings override in either direction when given explicitly — e.g. -merge=false disables merging even if the config file sets "merge": true. Category and rule severities are config-file only.

The remaining flags perform a one-off action and exit; run decolint -help for the full list:

Flag	Action
-config <path>	use the config file at <path> instead of auto-discovery
-init	write a new .decolint.jsonc listing every rule at its default severity
-rules	print the built-in rules as a Markdown table
-version	print version information
-help	print usage

Target platforms

Each rule optionally targets specific platforms (vscode, codespaces, ...); a rule with no target platform applies to every platform and always runs. By default, only those platform-agnostic rules run; pass -platform with a comma-separated list to also run rules scoped to specific platforms:

decolint -platform=vscode,codespaces

Merging

The Features a devcontainer.json references, and the base image it names, both contribute configuration of their own, which the Dev Container tooling merges into the effective configuration following the specification's merge logic. By default decolint lints only the raw file, so an issue introduced by a Feature or inherited from the base image (say, one that sets privileged: true or bind-mounts the Docker socket) goes unnoticed.

Enable merging to lint the merged configuration instead:

decolint -merge

This fetches every referenced Feature and resolves the base image, including any metadata in the base image's devcontainer.metadata label. A Feature or image that cannot be fetched is an error (exit code 2).

Merging also resolves the ${...} variables in the devcontainer.json first, so the reference a Feature or image is fetched by, and the values the rules see, match what the real tooling would use:

• ${localEnv:NAME} (and ${env:NAME}) resolves from the config file's localEnv map only — decolint never reads environment variables. A name missing from the map resolves to the default in ${localEnv:NAME:default}, or to the empty string.
• ${localWorkspaceFolder} resolves to the linted directory's absolute path, ${containerWorkspaceFolder} to the configuration's workspaceFolder (defaulting to /workspaces/<folder name>, or / for Docker Compose); each has a ...Basename variant.
• ${devcontainerId} resolves to a fixed placeholder with the format of a real id, since the real value exists only once a container is created.
• Anything else, including ${containerEnv:NAME}, resolves to the empty string.

A few limits apply:

• For Docker Compose, extends and include are resolved as docker compose config would, and later files override earlier ones. Compose interpolation uses its own bare ${NAME} syntax — not devcontainer.json's ${localEnv:NAME} — but reads the same values from the config file's localEnv map: an unset variable resolves to its default (${VAR:-default}) or the empty string, and a ${VAR:?} requirement on an unset variable is an error. Compose profiles, the COMPOSE_FILE environment variable, and .env files are not applied.
• Registries are accessed anonymously, so a private image that cannot be pulled that way counts as a fetch failure.

Output formats

By default, findings are printed one per line, with the rule's severity (error or warn):

.devcontainer/devcontainer.json:4:12: warn: image "ubuntu:latest" uses the "latest" tag; pin a specific version (no-image-latest)

Select a different output format to change this:

• text (default) — the one-line-per-finding format shown above.
• json — a JSON array of finding objects, for scripting:

  [{"path":".devcontainer/devcontainer.json","line":4,"col":12,"ruleId":"no-image-latest","message":"image \"ubuntu:latest\" uses the \"latest\" tag; pin a specific version","severity":"warn"}]

• github — GitHub Actions workflow commands, so findings show up as inline annotations on pull request diffs when decolint is run from a GitHub Actions workflow:

  ::warning file=.devcontainer/devcontainer.json,line=4,col=12,title=no-image-latest::image "ubuntu:latest" uses the "latest" tag; pin a specific version
  

• sarif — a SARIF 2.1.0 log, for upload to GitHub Code Scanning so findings appear as alerts in the repository's Security tab:

  - run: decolint -merge -format=sarif . > decolint.sarif
    continue-on-error: true # decolint exits 1 on findings; the upload must still run
  - uses: github/codeql-action/upload-sarif@v3
    with:
      sarif_file: decolint.sarif

  Run decolint from the repository root, so that the paths in the log resolve to files in the repository.

Every format reports paths relative to the directory decolint runs in, whichever way the linted directory was named on the command line. A file outside that directory is reported with its absolute path.

Exit codes

• 0 — no error-severity findings (there may still be warn findings)
• 1 — at least one error-severity finding was reported
• 2 — an error occurred (e.g. a file could not be parsed)

Enable deny-warnings (the -deny-warnings flag or "denyWarnings": true) to also fail (exit code 1) on warn-severity findings. Exit codes are unaffected by the output format.

Config file

Every setting from the Configuration table can be declared per project in a JSON/JSONC config file. Run decolint -init to generate a starting .decolint.jsonc in the current directory, ready to edit:

// .decolint.jsonc
{
  "platforms": ["vscode"],
  "categories": {
    "security": "error"
  },
  "rules": {
    "no-image-latest": "error",
    "pin-image-digest": "warn",
    "require-non-root": "off"
  }
}

categories sets the severity (error, warn, or off) of every rule in a category at once; rules sets an individual rule's severity and takes precedence over its category. localEnv maps names to the values ${localEnv:NAME} resolves to when merging; it is also the environment Compose-file ${NAME} interpolation reads (note the differing syntax):

{
  "localEnv": { "USERPROFILE": "/home/user" }
}

The remaining members mirror their flags: platforms, merge, denyWarnings, and format (see the Configuration table).

For the strictest configuration, enable every category:

// .decolint.jsonc
{
  "categories": {
    "security": "error",
    "reproducibility": "error",
    "style": "error"
  }
}

decolint looks for .decolint.jsonc, then .decolint.json, in the current directory; the first one found is used. Pass -config <path> to use a file at a different location instead. It is an error (exit code 2) if -config points at a file that doesn't exist or fails to parse, or if the config references an unknown rule ID or category name.

Rules

Every rule belongs to one category, which sets its severity unless overridden by a config file. A rule can also optionally target specific platforms (see Target platforms); a rule with no target platform applies to all platforms.

Rule categories

Only correctness runs by default; the rest are off until enabled:

• correctness (default error) — the configuration is invalid or does not behave as written.
• security (default off) — container runtime privileges and hardening.
• reproducibility (default off) — unpinned versions or digests that make the environment change over time.
• style (default off) — discouraged or legacy configuration that still works.

ID	Category	Platform	Description
conflicting-container-def	correctness	(all)	disallow a devcontainer.json that defines more than one of image, build, or dockerComposeFile
feature-install-script-not-executable	correctness	(all)	disallow a Feature's install.sh that lacks executable permission bits
id-dir-mismatch	correctness	(all)	disallow a Feature's or Template's id that does not match the name of its containing directory
invalid-semver	correctness	(all)	disallow a Feature's or Template's version that is not a valid semantic version
missing-build-dockerfile	correctness	(all)	disallow a devcontainer.json build object that is missing dockerfile
missing-compose-service	correctness	(all)	disallow a devcontainer.json that sets dockerComposeFile without service
missing-container-def	correctness	(all)	disallow a devcontainer.json that defines none of image, build, or dockerComposeFile
missing-feature-install-script	correctness	(all)	disallow a Feature directory without the required install.sh install script
missing-required-props	correctness	(all)	disallow a Feature's or Template's metadata that is missing a required property (id, version, or name)
missing-workspace-mount-folder	correctness	(all)	disallow a devcontainer.json using image or build that sets only one of workspaceMount or workspaceFolder
no-bind-mount	correctness	codespaces	disallow bind type entries in mounts, which GitHub Codespaces silently ignores except for the Docker socket
no-host-port-format	correctness	codespaces	disallow host:port entries in forwardPorts and portsAttributes, which GitHub Codespaces does not support
undefined-template-option	correctness	(all)	disallow a ${templateOption:...} reference to an option not declared in devcontainer-template.json
no-cap-add-all	security	(all)	disallow granting all Linux capabilities via an ALL entry in a devcontainer.json's or Feature's capAdd property, or a --cap-add=ALL entry in a devcontainer.json's runArgs
no-docker-socket-mount	security	(all)	disallow bind-mounting the host's Docker socket via a devcontainer.json's mounts or runArgs, which grants the container root-equivalent control over the host
no-privileged-container	security	(all)	disallow running the container in privileged mode via a devcontainer.json's or Feature's privileged property, or a --privileged entry in a devcontainer.json's runArgs
no-seccomp-override	security	(all)	disallow overriding the container runtime's default seccomp profile via a devcontainer.json's or Feature's securityOpt property, or a --security-opt seccomp=... entry in a devcontainer.json's runArgs
no-seccomp-unconfined	security	(all)	disallow disabling seccomp confinement via a devcontainer.json's or Feature's securityOpt property, or a --security-opt seccomp=unconfined entry in a devcontainer.json's runArgs
require-cap-drop-all	security	(all)	require an ALL entry in a devcontainer.json's capDrop property, or a --cap-drop=ALL entry in runArgs, dropping every Linux capability
require-no-new-privileges	security	(all)	require no-new-privileges to be set via a devcontainer.json's securityOpt property, or a --security-opt no-new-privileges... entry in runArgs
require-non-root	security	(all)	require remoteUser or, if unset, containerUser to be set to a non-root user
no-image-latest	reproducibility	(all)	disallow container images without an explicit tag or with the latest tag
pin-extension-version	reproducibility	vscode, codespaces	disallow a customizations.vscode.extensions entry without an explicit pinned version
pin-feature-version	reproducibility	(all)	disallow a Feature reference without an explicit version or with the latest version
pin-image-digest	reproducibility	(all)	disallow an image property that does not pin the image by content digest (e.g. image@sha256:...)
no-app-port	style	(all)	disallow the legacy appPort property in favor of forwardPorts
unused-template-option	style	(all)	disallow a Template option that no file in the Template references

Suppressing findings

Findings can be suppressed with comments in the configuration files:

• decolint-ignore-line — suppress findings on the same line, typically as a trailing comment
• decolint-ignore-next-line — suppress findings on the next line
• decolint-ignore-file — suppress findings in the whole file

Each directive optionally takes rule IDs, separated by commas or spaces; omitting them suppresses all rules. Block comments (/* ... */) work the same way.

// decolint-ignore-file no-app-port
{
  // decolint-ignore-next-line no-image-latest
  "image": "ubuntu:latest",
  "privileged": true // decolint-ignore-line
}

Verifying release artifacts

Each release includes a decolint-<version>-checksums.txt file listing the SHA-256 checksum of every binary, plus a decolint-<version>-checksums.txt.sigstore.json file: a Sigstore bundle containing the cosign keyless signature (signed via GitHub Actions OIDC) and its Rekor transparency log entry.

To verify a downloaded binary:

cosign verify-blob \
  --bundle decolint-<version>-checksums.txt.sigstore.json \
  --certificate-identity-regexp '^https://github\.com/bare-devcontainer/decolint/\.github/workflows/release\.yml@.*$' \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com \
  decolint-<version>-checksums.txt

sha256sum --ignore-missing -c decolint-<version>-checksums.txt

The first command confirms the checksums file was signed by this repository's release workflow; the second confirms the downloaded binary matches a checksum in that file.

Each binary's provenance can also be verified with gh attestation verify, using the build provenance attested during the release:

gh attestation verify decolint_<version>_<os>_<arch>.tar.gz \
  --repo bare-devcontainer/decolint

The container image carries the same kind of attestation:

gh attestation verify oci://ghcr.io/bare-devcontainer/decolint:<version> \
  --repo bare-devcontainer/decolint

Contributing

Rules are plain Go code and new ones are easy to add; see CONTRIBUTING.md for the development workflow and a walkthrough of adding a rule.

License

MIT