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>

builder.Dockerfile

@@ -0,0 +1,21 @@
+# CI builder image for Love2D projects: packaging tools + luac syntax check,
+# with love.js pre-fetched to /opt/lovejs.zip so `make web` does not hit
+# GitHub on every pipeline run (the Makefile falls back to curl elsewhere).
+#
+# Build & push (the CI runner is linux/amd64):
+#   docker build --platform linux/amd64 -f builder.Dockerfile \
+#     -t git.teletype.hu/internal/love-builder:latest .
+#   docker push git.teletype.hu/internal/love-builder:latest
+
+FROM alpine:3.22
+
+RUN apk add --no-cache make zip unzip curl git lua5.4 \
+ && ln -sf /usr/bin/lua5.4 /usr/bin/lua \
+ && ln -sf /usr/bin/luac5.4 /usr/bin/luac
+
+RUN curl -sSL https://github.com/2dengine/love.js/archive/refs/heads/master.zip \
+      -o /opt/lovejs.zip
+
+WORKDIR /workspace
+
+CMD ["/bin/sh"]