Update filename generation to replace ':' in image names

Fix: Replace colons and slashes in Docker image filenames during migration

Replace both colons (:) and slashes (/) with underscores (_) in generated 
filenames to avoid invalid filename errors when migrating Docker images to 
Podman. This prevents "payload does not match" errors from podman load.

- Changed tr '/' '_' to tr '/:' '_' in migrate_images()
- Ensures compatibility with images containing version tags (e.g., ghost:latest)
- Resolves issue with multi-part repository names (e.g., linuxserver/wireguard)

fly-to-podman.sh

@@ -12,7 +12,7 @@ migrate_images() {
         fi
 
         # Replace slashes in repository names with underscores for filenames
-        filename=$(echo "$image" | tr '/' '_').tar
+        filename=$(echo "$image" | tr '/:' '_').tar
 
         echo "Exporting $image..."
         docker save -o "$filename" "$image" &&