Run lint and tests as phases of the Dockerfile (closes #5)

The Dockerfile gets a lint phase on the pinned golangci-lint v2.12.2
image and a test phase on the Debian Go image. The tests run as an
unprivileged user, because root reads a file with mode 0000 and the
permission test then fails. The build stage copies a file from each
phase, so no build finishes unless both pass, and it no longer runs
make check. script/lint and script/test each build their phase,
uncached and tagged; script/cibuild bootstraps, runs script/check, then
builds the image. script/bootstrap no longer installs golangci-lint.
README.md and TODO.md describe the new setup.

Model: opus-5-5

script/check

@@ -1,6 +1,8 @@
 #!/bin/sh
 # script/check: run all checks (test, lint, fmt-check). Our own
-# extension to scripts-to-rule-them-all. Must not modify any files.
+# extension to scripts-to-rule-them-all. test and lint are Docker
+# phases; fmt-check is native, because a formatter writes the working
+# tree. Must not modify any files.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"