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 +1,20 @@
+# CI builder image for Godot projects: barichello/godot-ci (editor + web
+# export templates) with make/zip preinstalled, so the pipeline's build step
+# does not run apt-get on every run.
+#
+# Keep the tag in sync with the Godot version the projects use.
+#
+# Build & push (the CI runner is linux/amd64):
+#   docker build --platform linux/amd64 -f builder.Dockerfile \
+#     --build-arg GODOT_VERSION=4.6 \
+#     -t git.teletype.hu/internal/godot-builder:4.6 .
+#   docker push git.teletype.hu/internal/godot-builder:4.6
+
+ARG GODOT_VERSION=4.6
+FROM barichello/godot-ci:${GODOT_VERSION}
+
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends make zip jq \
+ && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /workspace