Name the CI build step after what the image runs (closes #175)

The workflow's build step was named "Build Docker image (runs make check)", but the image has never run make check. It is now named for what the image runs: make fmt-check, golangci-lint, make test and make build. No other step name or comment in the workflow names something it does not run. In the README, the Prerequisites bullet now says Docker is needed for make lint and so for make check, and the sentence about what runs on the host is limited to the steps make check runs.

Model: opus-5-5

.gitea/workflows/check.yml

@@ -33,5 +33,5 @@ jobs:
         # report success from cache.
         run: git rev-parse HEAD > .ci-fingerprint
 
-      - name: Build Docker image (runs make check)
+      - name: Build Docker image (runs make fmt-check, golangci-lint, make test, make build)
         run: script/cibuild

README.md

@@ -19,8 +19,8 @@ before deploying one.
 ### Prerequisites
 
 - Go 1.26.1+ (the version in `go.mod`)
-- Docker (for linting, for the test stage of the CI gate, and for
-  containerized deployment)
+- Docker (for `make lint` and so for `make check`, for the CI gate, and
+  for containerized deployment)
 
 golangci-lint is not a prerequisite and must not be installed on the
 host: `script/bootstrap` does not install it, and `make lint` runs the
@@ -3335,8 +3335,9 @@ linked, which is what lets it run on the Alpine runtime image.
 inside the image, so a build that succeeds is a repo that is formatted,
 linted, tested and compiled. `script/lint` also uses Docker
 (`Dockerfile.lint`, see Linting above), so `make lint` and `make check`
-run the same pinned linter version the gate does; only `script/test`
-and `script/fmt-check` run on the host.
+run the same pinned linter version the gate does; of the steps
+`make check` runs, only `script/test` and `script/fmt-check` run on the
+host.
 
 #### CI gate honesty