Gate the build on Docker lint and test phases (closes #40, closes #30)

Per the owner ruling on issue 40, linting and testing are phases of the
main Dockerfile rather than a separate lint file. script/lint and
script/test each build one phase by name with caching disabled, and the
final stage copies a harmless file from each so the image cannot be built
unless both passed — the ordering trick already in template-app-go's
Dockerfile, extended to the test phase. A phase that is not the last
stage is built only when something depends on it or --target names it, so
the gates are always invoked by name and the edges are kept. Every build
in script/ is tagged. No config verify step; fmt stays on the host. This
closes issue 30 too: a container has its own result cache and lock.

Model: opus-5

script/cibuild

@@ -1,10 +1,11 @@
 #!/bin/sh
-# script/cibuild: run the CI build. --no-cache because the checks are
-# RUN steps: on an unchanged tree Docker serves them from cache and the
-# build exits 0 having run nothing.
+# script/cibuild: run the CI build. script/check runs the gates and
+# builds the image; this repeats the build with the version, which
+# reuses the phases just built rather than re-running them.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
@@ -15,7 +16,10 @@ main() {
     # version without failing.
     version="$(git describe --tags --always --dirty 2>/dev/null || true)"
     [ -n "$version" ] || version="unknown"
-    docker build --no-cache --build-arg VERSION="$version" .
+    "$SCRIPT_DIR/check"
+    docker build \
+        --build-arg VERSION="$version" \
+        -t "$("$SCRIPT_DIR/projectname")" .
 }
 
 main "$@"