Set up nightly builds

Closes Maintain Test and Build images in this repo (#57 - closed)

Creates a docker image that can be built periodically (recommend nightly for now) and used as the basis for pipeline tests.

Because the image has the most recent dependencies installed, only new ones added in the MR have to be added. In most cases this is none. Then the time taken to run the test suite is reduced to the time taken to download this image and run the tests.

• Prior time to run test-linux : 12mins
• New time to run test-linux: <3mins