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 +1,17 @@
+# CI builder image for Ebitengine projects: Go toolchain + packaging tools.
+# Used by the `build` step in example-woodpecker.yaml instead of installing
+# zip/make/curl on every pipeline run.
+#
+# Build & push (the CI runner is linux/amd64):
+#   docker build --platform linux/amd64 -f builder.Dockerfile \
+#     -t git.teletype.hu/internal/ebitengine-builder:latest .
+#   docker push git.teletype.hu/internal/ebitengine-builder:latest
+
+# 1.26 covers every current game repo's go.mod directive (1.25.x, 1.26.x).
+FROM golang:1.26-alpine
+
+RUN apk add --no-cache git make zip curl
+
+WORKDIR /workspace
+
+CMD ["/bin/sh"]