Add postgreSQL database to THEO

[mirenz1]

In preparation for CARL's persistence #18 we need to add a postgreSQL database to the openDuT test environment THEO.

• [ ] add postgreSQL docker image to CARL setup (compose file)

[reimarstier]

This was also done in the context of #171 and can be found here.

[mbfm]

This is implemented on the persistence branch. Will be merged onto development when more progress has been made on #18.