Fix entrypoint to use exec so that SIGINT shutdown works

[kjsanger]

Add a healthcheck to the Dockerfile

[nerdstrike]

Well drat: encode/uvicorn#1579

[kjsanger]

Without this change, the container doesn't shut down cleanly. With it I get:

$ docker run -p 8081:8081 wsinpg/python-3.10-npg-porch-2.0.0:latest
WARNING: The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested
Starting PostgreSQL 15 database server: main.
INFO:     Will watch for changes in these directories: ['/app']
INFO:     Uvicorn running on http://0.0.0.0:8081 (Press CTRL+C to quit)
INFO:     Started reloader process [7] using StatReload
INFO:     Started server process [44]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     127.0.0.1:59070 - "GET / HTTP/1.1" 200 OK
^CINFO:     Shutting down
INFO:     Waiting for application shutdown.
INFO:     Application shutdown complete.
INFO:     Finished server process [44]
INFO:     Stopping reloader process [7]

So it is an improvement even if within the container the server isn't necessarily behaving well. I'm not sure that I care about that here because the Dockerfile is dev/test only.

[nerdstrike]

I guess so.