Adding Memcached container.

You have implemented a new container and would like to contribute it? Great! Here are the necessary steps.

• 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).