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_traefik/.kamal/hooks/pre-deploy

@@ -1,4 +1,7 @@
-kamal proxy boot_config set --publish false \
+set -e
+
+kamal proxy boot_config set --registry registry:4443 \
+                            --publish false \
                             --docker_options label=traefik.http.services.kamal_proxy.loadbalancer.server.scheme=http \
                                              label=traefik.http.routers.kamal_proxy.rule=PathPrefix\(\`/\`\) \
                                              sysctl=net.ipv4.ip_local_port_range=\"10000\ 60999\"