docker: report the real version in the image (closes #109)

The Dockerfile builder stage takes ARG VERSION (default `dev`) and passes
it to `make build` on the command line, which overrides the Makefile's
`git describe` default. script/docker computes the version from
`git describe` on the host and passes it as --build-arg VERSION, because
.dockerignore leaves .git out of the build context and `git describe`
inside the build only ever produced `dev`. A build that passes no
argument, such as script/cibuild, still reports `dev`.

Model: opus-5-5

Dockerfile

@@ -34,8 +34,11 @@ COPY . .
 # Run the tests - build fails if any test fails
 RUN make test
 
-# Build the binary
-RUN make build
+# Build the binary. .dockerignore leaves out .git, so `git describe` in
+# the Makefile cannot find the version here: script/docker passes it as
+# --build-arg VERSION, and a build that passes none reports `dev`.
+ARG VERSION=dev
+RUN make build VERSION="${VERSION}"
 
 # Runtime stage
 # alpine 3.21, 2026-02-28

Makefile

@@ -1,6 +1,8 @@
 .PHONY: all bootstrap setup build lint fmt fmt-check test check clean hooks docker
 
 BINARY := dnswatcher
+# `make build VERSION=...` overrides this; the Dockerfile does so, as the
+# image has no .git to describe.
 VERSION := $(shell git describe --tags --always --dirty 2>/dev/null || echo "dev")
 LDFLAGS := -X main.Version=$(VERSION)
 

README.md

@@ -480,7 +480,8 @@ them. We provide:
 - `script/check` — run test, lint, and fmt-check
 - `script/docker` — build the Docker image tagged via `script/projectname`, with
   `--no-cache-filter=lint,builder` so the lint stage and the builder stage,
-  which runs the tests, run on every invocation
+  which runs the tests, run on every invocation, and with the version from
+  `git describe` passed as `--build-arg VERSION`
 - `script/cibuild` — CI entrypoint: `docker build` with
   `--no-cache-filter=lint,builder`, so the lint stage and the builder stage,
   which runs the tests, run on every invocation, because a cached build lints
@@ -502,11 +503,14 @@ make clean      # Remove build artifacts
 
 ### Build-Time Variables
 
-Version is injected via `-ldflags`:
+`make build` sets the version with `-ldflags "-X main.Version=..."`, taking
+it from `git describe --tags --always --dirty`, or from `VERSION` when given
+on the command line (`make build VERSION=1.2.3`). The version appears in the
+startup log and in the health check response.
 
-```sh
-go build -ldflags "-X main.Version=$(git describe --tags --always)" ./cmd/dnswatcher
-```
+The Docker image has no `.git`, so the `Dockerfile` takes the version as
+`--build-arg VERSION`. `make docker` passes it; a plain `docker build`
+passes none, and that image reports `dev`.
 
 ---
 

TODO.md

@@ -19,6 +19,8 @@ Rationale, Design, TODO, License, Author) if any are still missing.
 
 # Completed Steps
 
+- 2026-10-01: the image built by `make docker` reports the `git describe`
+  version instead of `dev`, passed as `--build-arg VERSION` (closes #109).
 - 2026-10-01: wildcard CORS now applies only to the public routes, not to
   `/metrics`, and allows only the methods they serve (closes #100).
 - 2026-10-01: `internal/state` and `internal/watcher` no longer export test-only

script/docker

@@ -12,7 +12,15 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build --no-cache-filter=lint,builder -t "$("$SCRIPT_DIR/projectname")" .
+    # Own line: a failing command substitution inside an argument does
+    # not trip `set -e`, so the inline form degrades silently to an
+    # empty constant. VERSION is computed here because .dockerignore
+    # excludes .git, so `git describe` in a build stage cannot find it.
+    version="$(git describe --tags --always --dirty 2>/dev/null || true)"
+    [ -n "$version" ] || version="unknown"
+    docker build --no-cache-filter=lint,builder \
+        --build-arg VERSION="$version" \
+        -t "$("$SCRIPT_DIR/projectname")" .
 }
 
 main "$@"