Add support for IBM Db2

[eonu]

Adds support for containers created from the ibmcom/db2 image.

Notes

• The SQLAlchemy adapter ibm_db_sa used for Db2 does not yet support SQLAlchemy 2.0 (the devs are currently working on it), so this PR won't be able to go in until then, but thought I'd still get the review meanwhile. Alternatively, can I specify sqlalchemy<2 in the setup.py that I created?
• The container takes ages to start up, sometimes around 15 minutes on my machine, so I'm not sure we want it to run as part of the unit tests as it might hold everything up.
• The container must run in privileged mode.

Checks

• Create a new feature directory and populate it with the package structure described in the documentation. Copying one of the existing features is likely the best way to get started.
• Implement the new feature (typically in __init__.py) and corresponding tests.
• Add a line -e file:[feature name] to requirements.in and run make requirements. This command will find any new requirements and generate lock files to ensure reproducible builds (see the pip-tools documentation for details). Then run pip install -r requirements/[your python version].txt to install the new requirements.
• Update the feature README.rst and add it to the table of contents (toctree directive) in the top-level README.rst.
• Add a line [feature name] to the list of components in the GitHub Action workflow in .github/workflows/main.yml to run tests, build, and publish your package when pushed to the master branch.
• Rebase your development branch on master (or merge master into your development branch).