Add gitpod/workspace-elixir-postgresql image

[axonasif]

Is your feature request related to a problem? Please describe

"It would be helpful to many Elixir developers to have a gitpod/workspace-elixir-postgres which installs postgres so that updates to the database are saved in the /workspace of the instance as your gitpod/workspace-postgres does currently." - on here.

Describe the behaviour you'd like

Create a new combination of lang-elixir + tool-postgresql.

Describe alternatives you've considered

Additional context

The below is part of another feedback/suggestion but possibly doesn't require much consideration as with adding a new image:

• Install "esl-erlang" package for elixir image as well #1023