Podman fix: derive `--userns=keep-id` mapping from the remote user's actual UID/GID

まず、Dev Containers CLI が単純な --userns=keep-id 引数をどこで注入し、イメージ内の remoteUser をどのように解決しているかを追跡します。ホストの UID/GID が大きい場合と、ホストとコンテナーの ID が一致する場合について、生成される Podman 引数を検証し、どちらの場合も期待される workspace の権限が維持されることを確認します。

The problem

When running a dev container with rootless podman, the Dev Containers CLI injects a plain --userns=keep-id flag, which maps the host user id to the same UID number inside the container.

This breaks for high/out-of-range host UIDs (e.g. AD/SSSD users which have values like1400601154, which are not valid inside the container (they are out of range as the containers are typically given a 0-65535 range via subuid/subgid mapping). As a result the mapping fails silently and the container's vscode user remains at UID 1000, not matching the host user, and causing Permission denied on the workspace bind mount).

Current workaround

The current workaround is a fragile manual runArgs entry (--userns=keep-id:uid=1000,gid=1000). Since the host UID is unmappable to itself, we keep the 1000 id inside the container and map the host user to that. This allows one to use podman on Linux boxes with AD and thus high uid/gid combinations, as it removes the requirement for the exact host UID to be inside the container's range.

The request for improvement

Instead of injecting a plain --userns=keep-id, the CLI should resolve the remoteUser's actual UID/GID in the image and emit --userns=keep-id:uid=<remote-uid>,gid=<remote-gid>. This fixes the high-UID case with no regressions: when the host UID already equals the container user's UID, keep-id:uid=X,gid=X produces the identical mapping to plain keep-id, so existing setups are unaffected.