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

The workflow's build step said it runs make check, which the image
never does: it runs make fmt-check, golangci-lint, make test and make
build. The README now says Docker is needed for make lint and so for
make check, and scopes "only script/test and script/fmt-check run on
the host" 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