Add information about build only docker-compose file

Author: fsalvi

README.md

@@ -22,6 +22,8 @@ Make sure you are in the root directory of the cloned repository.
 You need to have [Docker Engine](https://docs.docker.com/engine/) and [Docker Compose](https://docs.docker.com/compose/) installed on your machine.
 Under macOS (Intel or Apple silicon), instead of installing [Docker Desktop](https://docs.docker.com/desktop/) you can also use [HomeBrew](https://brew.sh/) with [Colima](https://github.com/abiosoft/colima): `brew install colima docker docker-compose`.  
 UID and GID environment variables are needed to avoid docker from writing files as root in your directory.
+By default, docker-compose will use the file docker-compose.yml which will build the website and serve it on 0.0.0.0:4000 .
+If you just need to build the website, add ```-f docker-compose_build-only.yml```
 
 ```
 env UID="$(id -u)" GID="$(id -g)" docker-compose up

docker-compose_build-only.yml

@@ -0,0 +1,9 @@
+version: '2'
+
+services:
+  jekyll:
+    user: "${UID}:${GID}"
+    build: .
+    command: sh -c "chown $UID / && bundle exec jekyll build"
+    volumes:
+      - .:/srv/jekyll