Stamp the git tag or short commit in a plain docker build (closes #46)

A plain `docker build .` stamped `unknown` into the page footer:
.dockerignore left out .git, and the Dockerfile built without the -X
flags. The context now carries .git without .git/config, which can hold
a credential. The build stage takes the VERSION build argument when one
is given, otherwise `git describe --tags --always`, fails the build if
.git is present and no version comes out, and stamps it together with
the full commit the footer links to. `make build` now uses
`git describe` as well, and script/docker is the current shared copy.

Model: opus-5-5

Dockerfile

@@ -40,8 +40,23 @@ RUN go mod download && go mod vendor
 # installed above. The suite is offline (the live-feed test is opt-in).
 RUN make test
 
-# Build the binary with CGO enabled (required for sqlite3)
-RUN CGO_ENABLED=1 GOOS=linux go build -o /routewatch ./cmd/routewatch
+# Build the binary with CGO enabled (required for sqlite3). The version the
+# page footer shows is the VERSION build argument when one is given, otherwise
+# `git describe --tags --always` of the .git in the build context (git comes
+# with this image): the tag on a tagged commit, tag-N-gHASH after one, the
+# short commit when no tag is reachable. A context that carries .git and still
+# yields no version fails the build. The footer links to the full commit.
+ARG VERSION
+RUN version="${VERSION:-$(git describe --tags --always || echo unknown)}"; \
+    if [ -e .git ] && { [ -z "$version" ] || [ "$version" = dev ] || \
+        [ "$version" = unknown ]; }; then \
+        echo "no version could be derived although the build context carries .git" >&2; \
+        exit 1; \
+    fi; \
+    CGO_ENABLED=1 GOOS=linux go build -o /routewatch -ldflags "\
+        -X git.eeqj.de/sneak/routewatch/internal/version.GitRevision=$(git rev-parse --verify HEAD || echo unknown) \
+        -X git.eeqj.de/sneak/routewatch/internal/version.GitRevisionShort=$version" \
+        ./cmd/routewatch
 
 # Create source archive with vendored dependencies
 RUN tar --zstd -cf /routewatch-source.tar.zst \