Custom proxy image registry, repo and version

Use the --registry, --repository and --image_version options of
`kamal proxy boot_config set` to change the kamal-proxy image used.

We'll still insist that the image version is at least as high as the
minimum.

test/integration/docker/deployer/app_with_proxied_accessory/config/deploy.yml

@@ -41,4 +41,4 @@ accessories:
         interval: 1
         timeout: 1
         path: "/"
-
+drain_timeout: 2