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

When the dev container is opened from a git worktree, 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.

Resolve the common git dir on the host in initializeCommand.sh via
`git rev-parse --git-common-dir`, write the absolute path into
.devcontainer/.env, and have docker-compose.yml bind-mount it at the
same path inside the container. The worktree's `.git` file's gitdir
reference then resolves cleanly. For a main checkout this is a
no-op-shaped redundant mount of <repo>/.git at the same host path
inside the container; nothing breaks.

.devcontainer/.env is generated per-launch and is gitignored.

Author: yahonda

.devcontainer/docker-compose.yml

@@ -6,6 +6,7 @@ services:
     volumes:
       - ..:/workspaces/oracle-enhanced:cached
       - ./tzdata:/opt/tzdata:ro
+      - ${GIT_COMMON_DIR}:${GIT_COMMON_DIR}:cached
     command: sleep infinity
     network_mode: service:oracle
 

.devcontainer/initializeCommand.sh

@@ -4,6 +4,7 @@ set -euo pipefail
 
 IMAGE=gvenzl/oracle-free:latest
 SCRIPT_DIR=$(cd "$(dirname "$0")" && pwd)
+WORKSPACE_DIR=$(cd "$SCRIPT_DIR/.." && pwd)
 OUT_DIR="$SCRIPT_DIR/tzdata"
 
 docker pull "$IMAGE"
@@ -17,3 +18,16 @@ docker run --rm --entrypoint sh \
   -c 'cp "$ORACLE_HOME"/oracore/zoneinfo/timezlrg_*.dat /out/ && chmod a+r /out/*.dat'
 
 ls -1 "$OUT_DIR"
+
+# When opened from a git worktree, .git is a file whose `gitdir:` line points
+# at the main repo's .git/worktrees/<branch>. That host path is not visible
+# inside the dev container by default, so git commands inside the container
+# fail with "fatal: not a git repository". Resolve the common git dir on the
+# host and write it to .devcontainer/.env; docker-compose.yml bind-mounts it
+# at the same path inside the container so the gitdir reference resolves.
+GIT_COMMON_DIR=$(git -C "$WORKSPACE_DIR" rev-parse --git-common-dir)
+case "$GIT_COMMON_DIR" in
+  /*) ;;
+  *) GIT_COMMON_DIR=$(cd "$WORKSPACE_DIR/$GIT_COMMON_DIR" && pwd) ;;
+esac
+printf 'GIT_COMMON_DIR=%s\n' "$GIT_COMMON_DIR" > "$SCRIPT_DIR/.env"

.gitignore

@@ -15,3 +15,4 @@ ojdbc*.jar
 .byebug_history
 debug.log
 .devcontainer/tzdata/
+.devcontainer/.env