Make the CI gate execute the checks it reports on (closes #119)

The workflow writes a build-context fingerprint before calling script/cibuild, so a code commit invalidates the COPY layer of the lint and builder stages and the checks really run, while a docs-only commit still replays from cache. A superseding run also rewrites the exact failure/Has been cancelled status left on commits that were never tested to skipped, so cancellation no longer reads as red. script/cibuild itself is untouched.

Dockerfile

@@ -12,7 +12,11 @@ WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 
-# Copy source code
+# Copy source code. In CI the context also carries .ci-fingerprint, whose
+# value changes with every commit that touches the build context (see
+# .gitea/workflows/check.yml). That invalidates this layer, so the checks
+# below cannot report success by replaying a cached pass. Do not add it to
+# .dockerignore.
 COPY . .
 
 # Run formatting check and linter
@@ -36,7 +40,8 @@ WORKDIR /build
 COPY go.mod go.sum ./
 RUN go mod download
 
-# Copy source code
+# Copy source code, including the .ci-fingerprint cache barrier described in
+# the lint stage above.
 COPY . .
 
 # Run tests and build