[feature] Add Docker container-based testing environment for contributors

[CodingWithSaksham]

Is your feature request related to a problem? Please describe.
Setting up a local development environment for openwisp-radius currently requires installing several system dependencies (such as FreeRADIUS, Redis, database libraries, XMLSec, compiler toolchains, and other packages) before contributors can run the test suite.

Describe the solution you'd like

• Add a Dockerfile that installs all the dependencies required to develop and test openwisp-radius.
• Provide a Docker Compose configuration with a test container connected to the required services
• Add a make test target to the Makefile that builds (if necessary) and executes the test suite inside the Docker environment.
• Add a docker-entrypoint.sh to configure adding of superuser (similar in openwisp-controller)

[openwisp-companion]

This issue has been automatically assigned to @CodingWithSaksham who opened PR #743 to address it. 🎯