Document automated Docker build from git (#5237)

* Document automated Docker build from git
Fix https://github.com/FreshRSS/FreshRSS/issues/5236
Contributes to https://github.com/FreshRSS/FreshRSS/issues/4089

* Rolling

Docker/README.md

@@ -109,15 +109,16 @@ docker rm freshrss_old
 Building your own Docker image is especially relevant for platforms not available on our Docker Hub,
 which is currently limited to `x64` (Intel, AMD) and `arm32v7`.
 
-```sh
-# First time only
-git clone https://github.com/FreshRSS/FreshRSS.git
+> ℹ️ If you try to run an image for the wrong platform, you might get an error message like *exec format error*.
 
-cd FreshRSS/
-git pull --ff-only --prune
-docker build --pull --tag freshrss/freshrss:custom -f Docker/Dockerfile .
+Pick `#latest` (stable release) or `#edge` (rolling release) or a specific release number such as `#1.21.0` like:
+
+```sh
+docker build --pull --tag freshrss/freshrss:latest -f Docker/Dockerfile-Alpine https://github.com/FreshRSS/FreshRSS.git#latest
 ```
 
+> ℹ️ See an automated way to do that in our [Docker Compose](#docker-compose) section, leveraging a [git build context](https://docs.docker.com/build/building/context/#git-repositories).
+
 ## Development mode
 
 To contribute to FreshRSS development, you can use one of the Docker images to run and serve the PHP code,
@@ -297,6 +298,11 @@ volumes:
 services:
   freshrss:
     image: freshrss/freshrss:edge
+    # Optional build section if you want to build the image locally:
+    build:
+      # Pick #latest (stable release) or #edge (rolling release) or a specific release like #1.21.0
+      context: https://github.com/FreshRSS/FreshRSS.git#edge
+      dockerfile: Docker/Dockerfile-Alpine
     container_name: freshrss
     restart: unless-stopped
     logging:

Docker/freshrss/docker-compose.yml

@@ -7,7 +7,12 @@ volumes:
 services:
 
   freshrss:
-    image: freshrss/freshrss
+    image: freshrss/freshrss:latest
+    # Optional build section if you want to build the image locally:
+    build:
+      # Pick #latest (stable release) or #edge (rolling release) or a specific release like #1.21.0
+      context: https://github.com/FreshRSS/FreshRSS.git#latest
+      dockerfile: Docker/Dockerfile-Alpine
     container_name: freshrss
     hostname: freshrss
     restart: unless-stopped