.containerignore excludes .git, so go build's automatic VCS stamping produced
nothing and telesrv logged git_commit/git_branch/git_tree_state/build_time as
"unknown" on startup.
- Containerfile: accept GIT_COMMIT/GIT_BRANCH/GIT_TREE_STATE/BUILD_TIME build
args and pass them to the gramsrv build via -ldflags -X.
- build.sh: wrapper that fills those args from the current checkout and runs
podman build.
