Add build instrucions

ABOUT-FORK.md

@@ -30,7 +30,23 @@ Don't be overwhelmed, as commits are very granular.
 
 ### Builds
 
-Docker builds [are available here](https://hub.docker.com/r/minecon724/forgejo724) for amd64 and arm64.
+Docker builds [are available here](https://hub.docker.com/r/minecon724/forgejo724) for arm64.
+
+To build, [follow the official instructions](https://forgejo.org/docs/next/contributor/from-source/)
+
+To build for multiple architectures:
+1. Setup: (only do this once)
+   ```bash
+   docker buildx create --name mybuilder --use
+   docker buildx inspect --bootstrap
+   ```
+2. Build:
+   ```bash
+   docker buildx build --platform linux/amd64,linux/arm64 -t user/repo:tag --push .
+   ```
+   You can add more architectures, if you wish.
+
+You may get rate-limited (429) by code.forgejo.org. Try building for one architecture at a time.
 
 ### Changelog