doc: Update docker build process

doc/020_installation.rst

@@ -265,6 +265,12 @@ binary, you can get it with `docker pull` like this:
 
     $ docker pull restic/restic
 
+The container is also available on the GitHub Container Registry:
+
+.. code-block:: console
+
+    $ docker pull ghcr.io/restic/restic
+
 Restic relies on the hostname for various operations. Make sure to set a static
 hostname using `--hostname` when creating a Docker container, otherwise Docker
 will assign a random hostname each time.

doc/developer_information.rst

@@ -127,3 +127,5 @@ required argument is the new version number (in `Semantic Versioning
     go run helpers/prepare-release/main.go 0.14.0
 
 Checks can be skipped on demand via flags, please see ``--help`` for details.
+
+The build process requires ``docker``, ``docker-buildx`` and ``qemu-user-static-binfmt``.