Dockerfile: build psycopg2 in separate container

[pjonsson]

This avoids having to install development libraries in the production container, which makes the image ~300M smaller.

---

:books: Documentation preview :books:: https://datacube-explorer--561.org.readthedocs.build/en/561/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (d7fa351) 85.88% compared to head (1188536) 85.88%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #561 +/- ## ======================================== Coverage 85.88% 85.88% ======================================== Files 26 26 Lines 3457 3457 ======================================== Hits 2969 2969 Misses 488 488 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.