Move CI docker images to the s2n-tls repository

[goatgoose]

Problem:

Currently the docker images used by the CI are not stored in the s2n-tls Github repository. This makes it difficult for external users to run tests without opening a PR, since all of the test dependencies must be installed locally. This is especially problematic for tests with many dependencies such as the integration tests and sidetrail.

Solution:

The CI docker images should be moved to the s2n-tls repository, allowing users to build the images locally and run tests.