Add HEALTHCHECK instruction to Dockerfile.

The `Dockerfile` currently does not have a `HEALTHCHECK` instruction. Users of the Docker image currently need to add it manually (e.g. via Docker Compose). Instead, the `Dockerfile` itself should include the instruction.

To determine whether the service is healthy, one can use the [`pg_isready`](https://www.postgresql.org/docs/current/app-pg-isready.html) program, which is already part of the image. The program “checks the connection status of a PostgreSQL database server”.

Most of the default `HEALTHCHECK` [options](https://docs.docker.com/reference/dockerfile/#healthcheck) seem reasonable:
```
--interval=30s
--timeout=30s
--start-interval=5s
--retries=3
```

I set `--start-period=1s` because the default of `0s` is too quick for this program.

Users of the image can still override the `HEALTHCHECK` instruction if they want to customize the command or the options.

Author: fumoboy007