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Docker: port is already allocated

Error response from daemon: driver failed programming external connectivity on endpoint web:
Bind for 0.0.0.0:8080 failed: port is already allocated

Error starting userland proxy: listen tcp4 0.0.0.0:5432: bind: address already in use

Both mean the host port you're publishing (-p 8080:80, or ports: in Compose) is taken. The first usually means another container already publishes it; the second usually means a program on your machine does (often a locally installed Postgres, Redis or MySQL).

Find out which one

npx portcheck-cli 8080

portcheck recognizes container ports and names the container rather than showing com.docker.backend or docker-proxy:

$ portcheck kill 8080
Port 8080 is published by docker container "web" (nginx:alpine). Stop the container instead:
  docker stop web

It never kills Docker's own proxy process, which would take down Docker or every container it serves.

If it's another container

docker ps --filter publish=8080        # which container
docker stop web                        # or: docker compose down, in its project

A common cause is a Compose project started from another folder, or an old project whose containers were never stopped: docker ps shows them all.

If it's a program on your machine

A database or dev server installed directly on your machine:

npx portcheck-cli kill 5432

For databases run as services, stop the service instead of the process, or it will start again: brew services stop postgresql@17 on macOS, sudo systemctl stop postgresql on Linux. On Windows, use the Services app.

Or publish the container on another host port: -p 5433:5432.

Docker Desktop on Windows and WSL

Docker Desktop and WSL2 reserve ranges of ports on Windows. If nothing is listening but the bind still fails, the port is probably reserved; see Windows: a port is "forbidden" but nothing uses it.