Enable building log4net with docker container

[FreeAndNil]

Provide the following convenience for PMC members to review the project:

git checkout <release-tag>
docker build -t log4net-builder .
docker run log4net-builder

That is, a convenience to
build a Docker image containing all necessary dependencies to be able to build the project
an entry point to build the project using the created Docker image.

Proposed by @vy