Run lint and tests as Dockerfile phases built with --no-cache (closes #202)

script/check, cibuild, docker, lint, test, setup and install-precommit
are now the sneak/prompts main copies, unchanged: lint and test each
build their Dockerfile phase with --no-cache. The lint phase runs
golangci-lint from the image REPO_POLICIES.md names, with libvips-dev
from apt-get; the test phase runs the tests with a 90-second timeout;
the build stage depends on both. script/bootstrap installs the C
compiler and image libraries only with --cgo, which the test phase and
build stage pass, and refreshes the apt lists before its first apt
install. Dockerfile.lint and CHECK_EPOCH are gone, and make
docker-versioned and docker-test call the scripts. Without VERSION the
build stage still uses git describe, per issue 166.

Model: opus-5-5

script/check

@@ -1,7 +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.
-# Generic: usually needs no adaptation.
+# 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)"