Update INSTALL_DOCKER for GitHub Container Registry change

INSTALL_DOCKER.md

@@ -14,60 +14,74 @@ If you prefer to use the Alpine images, you can use the following tags:
 
 The docker images also include common Pony tools like [ponyup](https://github.com/ponylang/ponyup), [corral](https://github.com/ponylang/corral), and [changelog-tool](https://github.com/ponylang/changelog-tool).
 
+## Where to find images
+
+Images from 0.56.1 and later are available from GitHub Container Registry. Earlier images are hosted on DockerHub. To use an image from DockerHub, change the command below from the pattern:
+
+```bash
+docker pull docker://ghcr.io/ponylang/ponyc:TAG
+```
+
+to
+
+```bash
+docker pull ponylang/ponyc:TAG
+```
+
 ## Using Pony from Docker
 
 You'll need to install Docker using [the instructions here](https://docs.docker.com/engine/installation/). Then you can pull a pony docker image using the following command (where TAG is the tag you want to use)
 
 ```bash
-docker pull ponylang/ponyc:TAG
+docker pull docker://ghcr.io/ponylang/ponyc:TAG
 ```
 
 Then you'll be able to run `ponyc` to compile a Pony program in a given directory, running a command like this:
 
 ```bash
-docker run -v /path/to/my-code:/src/main ponylang/ponyc:TAG
+docker run -v /path/to/my-code:/src/main docker://ghcr.io/ponylang/ponyc:TAG
 ```
 
 If you're unfamiliar with Docker, remember to ensure that whatever path you provide for `/path/to/my-code` is a full path name and not a relative path, and also note the lack of a closing slash, `/`, at the *end* of the path name.
 
 Note that if your host doesn't match the docker container, you'll probably have to run the resulting program inside the docker container as well, using a command like this:
 
 ```bash
-docker run -v /path/to/my-code:/src/main ponylang/ponyc:TAG ./main
+docker run -v /path/to/my-code:/src/main docker://ghcr.io/ponylang/ponyc:TAG ./main
 ```
 
 To compile and run in one step run a command like this:
 
 ```bash
-docker run -v /path/to/my-code:/src/main ponylang/ponyc:TAG sh -c "ponyc && ./main"
+docker run -v /path/to/my-code:/src/main docker://ghcr.io/ponylang/ponyc:TAG sh -c "ponyc && ./main"
 ```
 
 ### Docker for Windows
 
 Pull an image as above:
 
 ```bash
-docker pull ponylang/ponyc:TAG
+docker pull docker://ghcr.io/ponylang/ponyc:TAG
 ```
 
 Share a local drive (volume), such as `c:`, with Docker for Windows, so that they are available to your containers. (Refer to [shared drives](https://docs.docker.com/docker-for-windows/#shared-drives) in the Docker for Windows documentation for details.)
 
 Then you'll be able to run `ponyc` to compile a Pony program in a given directory, running a command like this:
 
 ```bash
-docker run -v c:/path/to/my-code:/src/main ponylang/ponyc:TAG
+docker run -v c:/path/to/my-code:/src/main docker://ghcr.io/ponylang/ponyc:TAG
 ```
 
 Note the inserted drive letter. Replace with your drive letter as appropriate.
 
 To run a program, run a command like this:
 
 ```bash
-docker run -v c:/path/to/my-code:/src/main ponylang/ponyc:TAG ./main
+docker run -v c:/path/to/my-code:/src/main docker://ghcr.io/ponylang/ponyc:TAG ./main
 ```
 
 To compile and run in one step run a command like this:
 
 ```bash
-docker run -v c:/path/to/my-code:/src/main ponylang/ponyc:TAG sh -c "ponyc && ./main"
+docker run -v c:/path/to/my-code:/src/main docker://ghcr.io/ponylang/ponyc:TAG sh -c "ponyc && ./main"
 ```