Update dependency dynaconf to v3

See merge request infrablue/infrablue!21

Update dependency pytest-django to v4

See merge request infrablue/infrablue!39

Update python Docker tag to v3.9

See merge request infrablue/infrablue!37

Build docker image

See merge request infrablue/infrablue!38

This reverts commit dbbd146f.

Update dependency isort to v5

See merge request infrablue/infrablue!22

Update dependency Django to <3.2

See merge request infrablue/infrablue!35

Update dependency flake8-rst-docstrings to ^0.0.14

See merge request infrablue/infrablue!36