Lint static/js/ with ESLint in Docker (closes #120)

`REPO_POLICIES.md` binds the repo to a JavaScript styleguide, but nothing checked `static/js/`. ESLint, pinned by `package.json` and `yarn.lock`, now lints it with the styleguide's two checkable rules, `no-var` and `prefer-const`. It runs only in Docker on a digest-pinned node image: a `js-deps` stage installs ESLint and stays cached until the manifests change, and a `js-lint` stage runs it. `script/lint` builds `js-lint`, so `make lint` and `make check` fail on a violation, and the image build depends on it as on the repo's other checks. ESLint, node and yarn are not prerequisites, and `make lint` never uses a host copy.

Model: opus-5-5

script/lint

@@ -1,6 +1,8 @@
 #!/bin/sh
-# script/lint: run the linter. golangci-lint is never installed locally: it
-# runs via docker only, one way, everywhere — script/lint builds
+# script/lint: run the linters, golangci-lint over the Go code and then
+# ESLint over static/js/. Neither is ever installed locally.
+#
+# golangci-lint runs via docker only, one way, everywhere — script/lint builds
 # Dockerfile.lint, which COPYs the repo into the pinned golangci-lint image
 # and lints as a build step. This works even when the docker daemon is remote
 # and bind mounts are impossible, and it removes the host linter's shared
@@ -50,6 +52,20 @@ main() {
         echo "  still matches a stage in Dockerfile.lint." >&2
         exit 1
     fi
+
+    # ESLint runs in the Dockerfile's js-lint stage, which the image build
+    # runs too. It prints nothing on a pass, so there is no summary to look
+    # for. Instead the stage is named once, for both flags: --target fails
+    # on a name that matches no stage, so a rename cannot leave
+    # --no-cache-filter silently ignored. The js-deps stage, which installs
+    # ESLint, keeps its cache, so ESLint is not downloaded again.
+    js_stage=js-lint
+    docker build \
+        --target "$js_stage" \
+        --no-cache-filter="$js_stage" \
+        --progress=plain \
+        --output=type=cacheonly \
+        .
 }
 
 main "$@"