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/.kamal/hooks/pre-deploy

@@ -1,3 +1,6 @@
 #!/bin/sh
+set -e
+
+kamal proxy boot_config set --registry registry:4443
 echo "Deployed!"
 mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/pre-deploy