Add CI builder image and use it in the pipeline

builder.Dockerfile bakes the build step's dependencies into
git.teletype.hu/internal/* so the pipeline no longer installs
packages on every run.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

builder.Dockerfile

@@ -0,0 +1,15 @@
+# CI builder image for Phaser projects: Node (syntax check) + packaging
+# tools, so the pipeline's build step does not run apk on every run.
+#
+# Build & push (the CI runner is linux/amd64):
+#   docker build --platform linux/amd64 -f builder.Dockerfile \
+#     -t git.teletype.hu/internal/phaser-builder:latest .
+#   docker push git.teletype.hu/internal/phaser-builder:latest
+
+FROM node:22-alpine
+
+RUN apk add --no-cache make zip curl git
+
+WORKDIR /workspace
+
+CMD ["/bin/sh"]