Remove .buildpacks and set port in Dockerfile

.buildpacks

@@ -1 +0,0 @@
-https://github.com/jakeg/heroku-buildpack-bun

Dockerfile

@@ -19,6 +19,7 @@ COPY . .
 # Build the client (Vite) and server (TSC)
 # ensuring the environment variables are set for the build if needed
 ENV NODE_ENV=production
+ENV PORT=3000
 RUN bun --filter "*" build
 
 # Expose the port (Dokku will override PORT env var, but 3000 is a good default documentation)