feat(dind): implement POST /containers/{id}/attach for docker run

[luthermonson]

Summary

Docker CLI's docker run <image> (non-detached) issues three calls in sequence:

POST /containers/create        → returns container ID
POST /containers/{id}/attach   → HTTP/1.1 Upgrade hijacks the connection
POST /containers/{id}/start    → kicks off the task

We previously returned 501 Not Implemented for any /attach because the route in routeContainer fell through to handleNotImplemented. The CLI surfaced this as:

unable to upgrade to tcp, received 501

so even after the alpine:3.20 ref-qualifier fix landed, the second fault in the same docker run invocation kept ephpm's dind-based jobs broken. This PR implements the missing handler.

Implementation

Modeled on the existing handleExecStartHijack in pkg/dind/exec.go:

• pkg/dind/attach.go (new):

  • handleContainerAttach — verifies Upgrade+Connection headers, drains the request body, hijacks the conn, sends HTTP/1.1 101 UPGRADED, then waits on entry.started for the task to be running. Once started, tails entry.LogPath to the upgraded conn, multiplexed with stdcopyWriter when Tty=false.
  • waitForStart(reqCtx, entry, deadline) — blocks on entry.started, request-context cancellation, or a 60s deadline. Defensive polling fallback if started is somehow nil.
  • tailLogToWriter(logPath, dst, done) — generic file tailer driven by a chan struct{} so unit tests don't pull in containerd types. The handler bridges from Task.Wait()'s ExitStatus channel via a small goroutine.

• pkg/dind/containers.go:

  • containerEntry gains one field, started chan struct{}, initialized in handleContainerCreate and closed by handleContainerStart after task.Start() succeeds.
  • routeContainer now dispatches attach to the new handler.

Output framing

When the container was started without a TTY (Tty=false), every chunk goes back with Docker's stdcopy 8-byte header (stream-type[1] + padding[3] + size[4] big-endian, then payload). The Docker CLI demultiplexes those into its own stdout and stderr.

Because cio.LogFile merges container stdout and stderr into a single file, we emit everything on stream 1 (stdout) under non-TTY mode. Perfect-fidelity split would require splitting the log files at task-create time. Flagged in the file comment as a future-improvement.

Tests

pkg/dind/attach_test.go (7 cases):

• TestHandleContainerAttach_404Unknown — route lookup precedes hijack; unknown container id returns 404.
• TestHandleContainerAttach_400WithoutUpgrade — plain POST without Upgrade: tcp + Connection: upgrade headers returns 400 instead of dropping into the hijack path.
• TestWaitForStart_SignalUnblocks — closing started releases the wait promptly.
• TestWaitForStart_DeadlineExpires — never-closed started hits the deadline.
• TestWaitForStart_RequestCancelled — r.Context() cancellation short-circuits (handles client disconnect after /attach but before /start).
• TestTailLogToWriter_ForwardsThenExitsOnStatus — pre-seeded content streams, appended bytes stream, and the function exits after the done channel closes.
• TestStdcopyWriter_FrameLayout — regression assertion on the 8-byte header format the CLI demuxes.

Full pkg/dind suite green (TestPushHandlerEndToEnd, TestCleanup_DindNamespaces, all the existing exec/buildkit/auth tests still pass).

Out of scope (documented in the file comment)

• Stdin attach (docker run -i): hijacked conn is writable but the bytes go nowhere yet. Requires a containerd IO mode set at task-create time, which dind doesn't currently expose. The vast majority of docker run invocations in CI workloads (docker run alpine:3.20, docker run -d ..., build helpers, etc.) are non-interactive and unblocked by this PR.
• True stdout/stderr split: see the framing note above.

Test plan

• All pkg/dind unit tests pass locally with CGO_ENABLED=0 go test -tags containers_image_openpgp ./pkg/dind/....
• Live verification: trigger an ephpm CI job that does docker run alpine:3.20 sh -c "echo hi" inside dind, confirm the container runs, output streams back, exit code propagates. Pairs with the alpine:3.20 ref-qualifier fix.

Related

• Follows the fix/dind-qualify-tag-parse PR which fixed the first failure (docker run alpine:3.20 → invalid port ":3.20"). Both bugs surface in the same docker run invocation but have unrelated root causes. Either alone leaves the other still broken; they're separate PRs for reviewability, both needed to actually unblock the workflow.