Allow to override `compose.yaml` `patient-web.build` with `patient-web.image` for deployment

[make-github-pseudonymous-again]

So that one can deploy from a published image instead of sources.

See:

• https://github.com/docker/compose/issues/7231#issuecomment-1698733139
• https://stackoverflow.com/a/76913131
• https://forums.docker.com/t/use-build-and-image-at-the-same-time/126507/3
• https://docs.docker.com/compose/compose-file/13-merge/