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/check

@@ -1,14 +1,22 @@
 #!/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.
+# script/check: run all checks (test, lint, fmt-check) and then the
+# image build. Our own 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 "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     "$SCRIPT_DIR/test"
     "$SCRIPT_DIR/lint"
     "$SCRIPT_DIR/fmt-check"
+    # No --no-cache here: the two phases this build depends on were just
+    # built uncached above, so what it reuses is that run and not an
+    # older one.
+    cd "$ROOT"
+    docker build -t "$("$SCRIPT_DIR/projectname")" .
 }
 
 main "$@"