Add CI builder image and use it in the pipeline

builder.Dockerfile bakes the export step's dependencies into
git.teletype.hu/internal/love-builder and pre-fetches love.js to
/opt/lovejs.zip; the Makefile prefers the cached copy and falls
back to downloading from GitHub outside the image.

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

example-woodpecker.yaml

@@ -6,9 +6,8 @@ steps:
       - make ci-version
 
   - name: export
-    image: alpine
+    image: git.teletype.hu/internal/love-builder:latest
     commands:
-      - apk add --no-cache zip make curl unzip
       - make ci-export
 
   - name: upload