Allow overriding ENTRYPOINT from .gitlab-ci.yml

Currently, the only way to make use of a Docker image that sets ENTRYPOINT seems to be to create and publish another Docker image based off the original, but setting ENTRYPOINT to an empty array. (as discussed in gitlab-org/gitlab-ci-multi-runner!21) This becomes an issue when you have several third-party images you need to use (e.g., the 5 most recent releases of Node.js), especially if you also want to stay up to date with the latest versions of those images.

Instead, I'd like the ability to override ENTRYPOINT with an option in .gitlab-ci.yml. Similar functionality is already provided by docker-compose (using the entrypoint option), and docker run supports overriding ENTRYPOINT using the --entrypoint command-line option.