Stamp the git tag or short commit into the binary (closes #10)

rtnetmon had no version. main.Version is now set at link time and logged
in the first startup line. `make build` and `make dev` set it from
`git describe --tags --always` unless VERSION is given. The Dockerfile
takes it from the VERSION build argument when one is given, otherwise
from `git describe --tags --always` of the .git the build context now
carries, and fails the build if the context carries .git and no version
comes out. .dockerignore follows the canonical copy: .git is sent,
.git/config, which can hold a credential, is not.

Model: opus-5-5

Dockerfile

@@ -34,4 +34,18 @@ WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
-RUN CGO_ENABLED=0 go build -trimpath -o /rtnetmon ./cmd/rtnetmon/
+# The version stamped into the binary: the VERSION build argument when one
+# is given, otherwise `git describe --tags --always` of the .git the build
+# context carries: the tag on a tagged commit, tag-N-gHASH on a commit after
+# one, the short commit when no tag is reachable. git ships in this base
+# image. A context that carries .git and still yields no version fails the
+# build. With neither, as from a source tarball, the binary reports dev.
+ARG VERSION
+RUN version="${VERSION:-$(git describe --tags --always)}"; \
+    if [ -e .git ] && { [ -z "$version" ] || [ "$version" = dev ] || \
+        [ "$version" = unknown ]; }; then \
+        echo "no version could be derived although the build context carries .git" >&2; \
+        exit 1; \
+    fi; \
+    CGO_ENABLED=0 go build -trimpath -ldflags="-X main.Version=${version:-dev}" \
+        -o /rtnetmon ./cmd/rtnetmon/