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: golang:1.25.6-alpine
+    image: git.teletype.hu/internal/ebitengine-builder:latest
     commands:
-      - apk add --no-cache zip make curl
       - make ci-export
 
   - name: artifact