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

@@ -6,28 +6,18 @@ steps:
       - 'make ci-version'
 
   - name: lint
-    image: alpine
+    image: git.teletype.hu/internal/tic80-luatools:latest
     commands:
-      - 'apk add --no-cache make 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'
       - 'make ci-lint'
 
   - name: minify
-    image: alpine
+    image: git.teletype.hu/internal/tic80-luatools:latest
     commands:
-      - 'apk add --no-cache make lua5.4 curl'
-      - 'ln -sf /usr/bin/lua5.4 /usr/bin/lua'
       - 'make ci-minify'
 
   - name: docs
-    image: alpine
+    image: git.teletype.hu/internal/tic80-luatools:latest
     commands:
-      - 'apk add --no-cache make lua5.4 lua5.4-dev luarocks gcc musl-dev zip'
-      - 'ln -sf /usr/bin/lua5.4 /usr/bin/lua'
-      - 'ln -sf /usr/bin/luarocks-5.4 /usr/bin/luarocks'
-      - 'luarocks install ldoc'
       - 'make ci-docs'
 
   - name: export