Allow docker entrypoint to execute psql command in shell script if PGHOST is set

[mishaschwartz]

This resolves #318 by temporarily making the psql command an alias for docker_process_sql and exporting it so that it can be used in executed shell scripts.

This change is only applied for the duration of the docker_process_init_files function so it should not affect any other calls to psql in the container.

To test this:

docker run --rm -e 'POSTGRES_USER=username' -e 'PGUSER=username' -e 'POSTGRES_PASSWORD=password' -e 'POSTGRES_DB=postgis' -e 'PGDATABASE=postgis' -e 'PGHOST=localhost' ghcr.io/stac-utils/pgstac:v0.9.1 postgres

The above command should run with no failures

[mishaschwartz]

See #318