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/setup.sh

@@ -20,6 +20,7 @@ push_image_to_registry_4443() {
 install_kamal
 push_image_to_registry_4443 nginx 1-alpine-slim
 push_image_to_registry_4443 busybox 1.36.0
+push_image_to_registry_4443 basecamp/kamal-proxy v0.8.7
 
 # .ssh is on a shared volume that persists between runs. Clean it up as the
 # churn of temporary vm IPs can eventually create conflicts.