Update README.md for ghcr.io docker images (#450)

Pebble is now being published to the Github container registry as part
of the release process.

Update the README to link to it instead of docker hub. Fix the docker
examples to use ghcr.io.

Remove the note about multi-stage builds requiring docker ce 17.05 -
this is now a widely used feature, and the mentioned Docker version is 7
years old, so we shouldn't need a special note here.

README.md

@@ -121,9 +121,8 @@ for more information.
 
 #### Prebuilt Docker Images
 
-If you would prefer not to use the provided `docker-compose.yml`, or to build
-container images yourself, you can also use the [published
-images](https://hub.docker.com/r/letsencrypt/pebble/).
+Pebble releases are published as Docker images to the
+[Github Container Registry](https://github.com/orgs/letsencrypt/packages?repo_name=pebble)
 
 With a docker-compose file:
 
@@ -132,7 +131,7 @@ version: '3'
 
 services:
  pebble:
-  image: letsencrypt/pebble
+  image: ghcr.io/letsencrypt/pebble:latest
   command: pebble -config /test/my-pebble-config.json
   ports:
     - 14000:14000  # ACME port
@@ -146,13 +145,11 @@ services:
 With a Docker command:
 
 ```bash
-docker run -e "PEBBLE_VA_NOSLEEP=1" letsencrypt/pebble
+docker run -e "PEBBLE_VA_NOSLEEP=1" ghcr.io/letsencrypt/pebble
 # or
 docker run -e "PEBBLE_VA_NOSLEEP=1" --mount src=$(pwd)/my-pebble-config.json,target=/test/my-pebble-config.json,type=bind letsencrypt/pebble pebble -config /test/my-pebble-config.json
 ```
 
-**Note**: The Pebble dockerfile uses [multi-stage builds](https://docs.docker.com/develop/develop-images/multistage-build/) and requires Docker CE 17.05.0-ce or newer.
-
 ### Default validation ports
 
 To make it easier to test ACME clients and run challenge response servers