ci(config): add GHCR image publish workflow

Publish frontend, backend, and remote-gateway images to GHCR on
pushes to main and manual runs. Also update docker-compose to pull
the three service images from ghcr.io instead of Docker Hub.

docker-compose.yml

@@ -1,6 +1,6 @@
 services:
   frontend:
-    image: heavrnl/nexus-terminal-frontend:latest
+    image: ghcr.io/micah123321/nexus-terminal-frontend:latest
     container_name: nexus-terminal-frontend
     ports:
       - "18111:80"
@@ -11,7 +11,7 @@ services:
       - nexus-terminal-network
 
   backend:
-    image: heavrnl/nexus-terminal-backend:latest
+    image: ghcr.io/micah123321/nexus-terminal-backend:latest
     container_name: nexus-terminal-backend
     env_file:
       - .env
@@ -24,7 +24,7 @@ services:
       - nexus-terminal-network
 
   remote-gateway:
-    image: heavrnl/nexus-terminal-remote-gateway:latest
+    image: ghcr.io/micah123321/nexus-terminal-remote-gateway:latest
     container_name: nexus-terminal-remote-gateway
     environment:
       GUACD_HOST: guacd