cibuild: build with --no-cache like script/docker (closes #89)

A bare `docker build .` served the Dockerfile's check steps from the
layer cache on an unchanged tree and exited 0 without running them.
script/cibuild now computes the version on its own line and runs the
same build command as script/docker and the shared policy, --no-cache
included, so every CI build runs the checks. README.md describes
script/cibuild accordingly.

Model: opus-5-5

README.md

@@ -23,8 +23,9 @@ javascript library is planned.
 
 # Build Status
 
-CI runs via `script/cibuild` (`docker build .`), which executes `make check`
-(formatting, linting, tests). The `main` branch must always be green.
+CI runs `script/cibuild`, which builds the Docker image with `--no-cache`, so
+the formatting, lint and test steps in the `Dockerfile` run on every build. The
+`main` branch must always be green.
 
 # Entrypoints
 
@@ -56,8 +57,8 @@ provide:
   Docker lint stage, whose image has no node
 - `script/check` — run `script/test`, `script/lint`, and `script/fmt-check`
 - `script/docker` — build the Docker image tagged with the project name
-- `script/cibuild` — CI entrypoint: `docker build .` (the Dockerfile runs the
-  checks)
+- `script/cibuild` — CI entrypoint: builds the image with the same command as
+  `script/docker`, uncached, so the checks in the Dockerfile run every time
 - `script/precommit` — pre-commit checks: `go mod tidy` verification, then
   `script/check`
 - `script/install-precommit` — install the git pre-commit hook that runs