Make git work in the dev container when opened from a worktree

[yahonda]

Summary

When the dev container is opened from a git worktree (e.g. <repo>/.worktrees/<branch>/), the worktree's .git is a file whose gitdir: line points at a host path under the main repo's .git/worktrees/<branch>. That host path is not visible inside the container by default, so git inside the container fails with fatal: not a git repository: (null) — VS Code's safe.directory probe, credential.helper setup, and any developer git command all hit this. The dev container build itself completes, but the in-container git workflow is broken.

initializeCommand.sh already runs on the host, where git is available. Resolve the common git dir via git rev-parse --git-common-dir, write the absolute path into .devcontainer/.env, and have docker-compose.yml bind-mount that host path at the same path inside the container. The worktree's .git file's gitdir: reference then resolves cleanly.

For a main checkout the host path is <repo>/.git, and mounting it at <repo>/.git inside the container is a redundant no-op-shaped mount that doesn't conflict with the existing workspace mount.

Changes

• .devcontainer/initializeCommand.sh — after the TZ extraction, compute GIT_COMMON_DIR and write .devcontainer/.env.
• .devcontainer/docker-compose.yml — bind-mount ${GIT_COMMON_DIR}:${GIT_COMMON_DIR}:cached on the app service. docker-compose reads .env from the directory containing docker-compose.yml automatically.
• .gitignore — ignore .devcontainer/.env (generated per launch).

Test plan

• Open the dev container from a worktree (<repo>/.worktrees/<branch>/) and confirm:
  • The dev container builds without "fatal: not a git repository" lines during safe.directory and credential.helper setup.
  • In an interactive terminal inside the container: git status succeeds and git rev-parse --show-toplevel prints /workspaces/oracle-enhanced.
• Open the dev container from the main checkout (<repo>/) and confirm the same — the extra bind mount should be harmless.