Check formatting on the latest Go only, the toolchain that defines it

Dockerfile

@@ -1,9 +1,10 @@
 # The merge gate: `docker build .` fails if vet, formatting or tests fail, and CI
 # runs exactly this. Day-to-day, prefer `docker compose run` (bind-mounts source,
-# no rebuilds). GO_VERSION defaults to the latest stable Go; override it to test
-# the lowest supported version: docker build --build-arg GO_VERSION=1.22.12 .
+# no rebuilds). GO_VERSION defaults to the latest stable Go; the lowest supported
+# version builds the `portable` stage, which every supported Go must pass:
+# docker build --build-arg GO_VERSION=1.22.12 --target portable .
 ARG GO_VERSION=1.27.1
-FROM golang:${GO_VERSION}
+FROM golang:${GO_VERSION} AS portable
 
 WORKDIR /app
 
@@ -14,6 +15,8 @@ RUN go mod download
 COPY . .
 RUN go vet ./... && \
 	go run github.com/fzipp/gocyclo/cmd/gocyclo@v0.6.0 -over 14 -ignore _test . && \
-	{ unformatted="$(gofmt -l .)"; test -z "$unformatted" || \
-	  { echo "unformatted files:"; echo "$unformatted"; exit 1; }; } && \
 	go test -race ./...
+
+FROM portable
+RUN unformatted="$(gofmt -l .)"; test -z "$unformatted" || \
+	{ echo "unformatted files:"; echo "$unformatted"; exit 1; }