Error: port is already allocated

Port already allocated errors in CI stem from parallel Docker container launches competing for the same host ports. Unlike the similar Node.js EADDRINUSE error, Docker's port allocation error happens in the container runtime before your application code runs — the host port in -p 8080:8080 is claimed by the first container, and subsequent containers requesting the same host port fail immediately at startup. GitHub Actions matrix jobs run concurrently by default. If your integration test matrix spins up three test jobs, each running docker run -p 5432:5432 postgres:15, two of the three will fail because only one process can bind host port 5432. The solution: use different host port ranges per matrix job (-p 5432:5432, -p 5433:5432, -p 5434:5432) or use Docker's dynamic allocation by omitting the host port: -p 5432 assigns the next available host port, retrievable with docker port container-name 5432. Docker Compose solves this more cleanly: running each test suite with a different project name (docker-compose -p job-$GITHUB_RUN_ID-$MATRIX_INDEX up) uses isolated networks per project. When connecting to services through Docker's internal network by service name rather than by host port mappings, containers do not compete for host ports at all — services are only reachable container-to-container via the Compose network bridge.