run initdb if the postgres data dir is empty

[johnsmyth]

Is your feature request related to a problem? Please describe.
When running from docker, you may want to pre-install steampipe binaries and plugins in the container image and run them read only, but the data directory must be read write. Currently, if you mount that dir as a docker bind mount, or a kube volume, steampipe dant start because the data dir is empty.

Describe the solution you'd like
run initdb to initialize and empty db in the data dir if the postgres data dir is empty.