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/lint

@@ -1,37 +1,23 @@
 #!/bin/sh
-# script/lint: run golangci-lint over the whole tree. This is the only
-# way the linter is run, everywhere; it is never installed on the host.
+# script/lint: run the linter. Linting is a phase of the Dockerfile and
+# this builds that phase alone; the linter is never installed or run on
+# a developer host, where a shared result cache and a host-global lock
+# make its answer untrustworthy.
 #
-# Inside a container it runs the linter. Anywhere else it builds
-# Dockerfile.lint, whose last step runs this script again inside that
-# container.
-#
-# Dockerfile.lint and the Dockerfile lint stage set container=docker
-# (the systemd convention for marking a container) to say where we are.
-# /.dockerenv cannot: it is missing inside build steps, and present on
-# hosts that are themselves containers.
+# The phase is not the last stage in the file, so it is built only when
+# --target names it. --no-cache because a cached lint layer is a lint
+# that did not run. The tag makes each build replace the previous image
+# instead of leaving a dangling one behind.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    if [ "${container:-}" = docker ]; then
-        # `golangci-lint config verify` is not run: it fetches its JSON
-        # schema over an unpinned live HTTPS call, which REPO_POLICIES.md
-        # forbids.
-        echo "Running linter..."
-        golangci-lint run --config .golangci.yml ./...
-    else
-        # A new CACHEBUST on every run means the lint step is never
-        # served from cache (see Dockerfile.lint). The cacheonly output
-        # leaves no image behind.
-        docker build \
-            --progress=plain \
-            --build-arg CACHEBUST="$(date +%s)-$$" \
-            --output=type=cacheonly \
-            -f Dockerfile.lint .
-    fi
+    docker build --no-cache \
+        --target lint \
+        -t "$("$SCRIPT_DIR/projectname")-lint" .
 }
 
 main "$@"