chore: add Docker image for docs and Uncloud Compose deployment (dogfooding!)

docs/.dockerignore

@@ -0,0 +1,14 @@
+# Ignore everything and use an allowlist of what is allowed to not package any secrets by accident.
+*
+
+# Allow files and directories.
+!docs/
+!src/
+!static/
+!pkg/
+!Caddyfile
+!*.js
+!*.json
+
+# Ignore unnecessary files inside allowed directories.
+**/.DS_Store