split authkey setup script and invoke during poststart

[rajsinghtech]

The devcontainer was failing to authenticate automatically with TS_AUTH_KEY when env var was setup. The root cause was a timing issue:

• Build Phase: Feature installs and entrypoint runs (secrets NOT available)
• Runtime Phase: GitHub Codespaces injects secrets (too late for entrypoint)

This meant TS_AUTH_KEY was undefined when the authentication logic ran.
Changes

• Added postCreateCommand to run auth script
• Removed auth logic from entrypoint
• Created new script for authentication

fixes: #53

[calebshay]

Just going to throw it out there that this worked for me as well. Would be nice to get it into the latest release.

[calpoy]

In my Codespace, the Feature wasn’t seeing TS_AUTH_KEY or CODESPACE_NAME during the entrypoint either, even though both were available later in interactive shells.

Adding the following to the consuming repository’s devcontainer.json made the existing Feature work without any modifications:

"containerEnv": {
  "TS_AUTH_KEY": "${localEnv:TS_AUTH_KEY}",
  "CODESPACE_NAME": "${localEnv:CODESPACE_NAME}"
}

After doing this, the Feature successfully auto-authenticated with Tailscale and used CODESPACE_NAME as the hostname.