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>

example-woodpecker.yaml

@@ -6,9 +6,8 @@ steps:
       - 'make ci-version'
 
   - name: build
-    image: debian:stable-slim
+    image: git.teletype.hu/internal/c64-builder:latest
     commands:
-      - 'apt-get update && apt-get install -y --no-install-recommends make acme'
       - 'make ci-build'
 
   - name: artifact