build: stamp git metadata into the container image

.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.

build.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# Build the owpengram-server container image, stamping the current git state
+# into the binary (shows up in telesrv's startup log as git_commit/git_branch/
+# git_tree_state/build_time). .containerignore excludes .git, so go build's
+# automatic VCS stamping can't see the repo - the values are passed in here.
+#
+# Usage:  ./build.sh [extra podman build args...]
+#   IMAGE=my/tag ./build.sh            # override the image tag (default: owpengram-server)
+set -euo pipefail
+cd "$(dirname "$0")"
+
+IMAGE="${IMAGE:-owpengram-server}"
+
+podman build \
+  --build-arg GIT_COMMIT="$(git rev-parse HEAD)" \
+  --build-arg GIT_BRANCH="$(git rev-parse --abbrev-ref HEAD)" \
+  --build-arg GIT_TREE_STATE="$(git diff --quiet && echo clean || echo dirty)" \
+  --build-arg BUILD_TIME="$(date -u +%Y-%m-%dT%H:%M:%SZ)" \
+  -t "$IMAGE" \
+  -f Containerfile \
+  "$@" \
+  .