Validate containerization with test-build

[MatthiasHinz]

Expands on an idea mentioned in https://github.com/o2r-project/containerit/issues/6#issuecomment-273216858

After a session (or workspace or something else) was containerized, a user may be able to test-build an image in order to verify that

(1) The build is successful (2) The local session matches the dockerized session (3) More ideas?

1 and 2 can be achieved analog to tests/testthat/test_sessioninfo_reproduce.R, by turning the test into feature