Docker: 'Bind for 0.0.0.0:8080 failed: port is already allocated' (or 'address already in use') when starting a container or docker compose up. How do I find what holds the port and fix it?

Cause: publishing a port (-p 8080:80 or ports: in compose) makes Docker bind that host port. The error means something already uses it: often another container (an old one from a previous run, a second compose project, or a stopped-but-not-removed setup that's still running), sometimes a process on the host (a local dev server, database, or another proxy). On Docker Desktop, a crashed run can leave a stale binding until Docker restarts. Find the holder: - Containers: docker ps --filter publish=8080 (or docker container ls -a and look at the PORTS column; in compose, docker compose ls lists other projects). - Host processes: Linux/macOS: sudo lsof -iTCP:8080 -sTCP:LISTEN or ss -ltnp 'sport = :8080'; Windows: netstat -ano | findstr :8080, then look up the PID. Fix: 1. If it's an old container you don't need: docker stop <name> && docker rm <name> (or docker compose down in that project's directory). 2. If both need to run, publish a different host port: -p 8081:80, or in compose ports: ["8081:80"]. The container's internal port can stay the same. 3. Avoid fixed host ports where you don't need them: -p 80 (or ports: ["80"]) lets Docker pick a free host port; docker port <container> shows which. 4. If a host service holds the port, stop it or move it. 5. If nothing shows up but the error persists, restart Docker (Docker Desktop: Restart; Linux: sudo systemctl restart docker) to clear a stale binding. 6. Bind only where needed: -p 127.0.0.1:8080:80 keeps a dev service off the network, which also avoids conflicts with services bound to other interfaces. Verify: docker ps shows the container running with the expected PORTS mapping, and curl localhost:<port> reaches it. Sources: https://docs.docker.com/engine/network/port-publishing/ https://docs.docker.com/reference/cli/docker/container/ls/