Error: COPY failed: file not found

Docker volume and bind-mount permissions are a consistent source of Dockerfile Copy File Not Found on Linux hosts where user ID mapping is not automatic. Unlike macOS and Windows (which use a VM), Docker on Linux shares the host kernel and filesystem directly. When a container runs as a non-root user (best practice for security), files created by the host user may not be writable by the container user if the UIDs do not match. Conversely, files created by the container may be owned by an unknown UID on the host. The fix: use --user $(id -u):$(id -g) in docker run to run the container with the host user's UID/GID. For persistent data directories, create them on the host first with the correct ownership before mounting. For named volumes managed by Docker, use an entrypoint script to chown the volume directory to the container user at startup. Docker Compose's user: "${UID}:${GID}" option reads environment variables — set these in a .env file or pipeline step to ensure portability.