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”.

The default `HEALTHCHECK` [options](https://docs.docker.com/reference/dockerfile/#healthcheck) seem reasonable:
```
--interval=30s
--timeout=30s
--start-period=0s
--start-interval=5s
--retries=3
```

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

Author: fumoboy007

12/alpine3.19/Dockerfile

@@ -248,5 +248,7 @@ STOPSIGNAL SIGINT
 # documentation at https://www.postgresql.org/docs/12/server-start.html notes
 # that even 90 seconds may not be long enough in many instances.
 
+HEALTHCHECK CMD pg_isready || exit 1
+
 EXPOSE 5432
 CMD ["postgres"]

12/alpine3.20/Dockerfile

@@ -219,5 +219,7 @@ STOPSIGNAL SIGINT
 # documentation at https://www.postgresql.org/docs/12/server-start.html notes
 # that even 90 seconds may not be long enough in many instances.
 
+HEALTHCHECK CMD pg_isready || exit 1
+
 EXPOSE 5432
 CMD ["postgres"]