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 +1,18 @@
+# CI builder image for C64 projects: ACME assembler + make, so the
+# pipeline's build step does not run apt-get on every run. The VICE
+# emulator is a local-dev tool only and is intentionally not included.
+#
+# Build & push (the CI runner is linux/amd64):
+#   docker build --platform linux/amd64 -f builder.Dockerfile \
+#     -t git.teletype.hu/internal/c64-builder:latest .
+#   docker push git.teletype.hu/internal/c64-builder:latest
+
+FROM debian:stable-slim
+
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends make acme zip curl ca-certificates \
+ && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /workspace
+
+CMD ["/bin/bash"]