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 +1,24 @@
+# CI toolbox image for the TIC-80 pipeline's lint / minify / docs steps:
+# Lua 5.4 with luacheck and ldoc preinstalled, so the pipeline does not
+# run luarocks (and a C compiler install) on every run.
+#
+# The export step keeps using the separate TIC-80 Pro image
+# (git.teletype.hu/internal/tic80pro, built from the tic80pro-dockerfile
+# repo) — this image only covers the Lua tooling around it.
+#
+# Build & push (the CI runner is linux/amd64):
+#   docker build --platform linux/amd64 -f builder.Dockerfile \
+#     -t git.teletype.hu/internal/tic80-luatools:latest .
+#   docker push git.teletype.hu/internal/tic80-luatools:latest
+
+FROM alpine:3.22
+
+RUN apk add --no-cache make zip curl git lua5.4 lua5.4-dev luarocks gcc musl-dev \
+ && ln -sf /usr/bin/lua5.4 /usr/bin/lua \
+ && ln -sf /usr/bin/luarocks-5.4 /usr/bin/luarocks \
+ && luarocks install luacheck \
+ && luarocks install ldoc
+
+WORKDIR /workspace
+
+CMD ["/bin/sh"]