fix: resolve relative bind mounts against code dir for git-based compose deploys

Compose deployments using a git provider (GitHub/GitLab/Gitea/Bitbucket/
custom Git) clone the repo verbatim, so composePath can point into a
subdirectory (e.g. deploy/docker-compose.yml). Docker Compose resolves
relative bind mounts (../files) against the compose file's own directory,
not against the code/ dir where file mounts are actually written, so the
mount silently binds an empty directory instead of the configured content.

Pin --project-directory to code/ when building the docker compose command
so relative mounts always resolve the same way raw deployments already do.

Fixes #5181