Fix postgres user

[dappnodedev]

This PR is causing the following error in the postgres container:

Data page checksums are disabled.

initdb: error: could not access directory "/var/lib/postgresql/data": Permission denied
chmod: /var/lib/postgresql/data: Operation not permitted
The files belonging to this database system will be owned by user "postgres".
This user must also own the server process.

initdb: error: could not access directory "/var/lib/postgresql/data": Permission denied
The database cluster will be initialized with locale "en_US.utf8".
The default database encoding has accordingly been set to "UTF8".
The default text search configuration will be set to "english".

Data page checksums are disabled.