WIP: Add ability to specify environment variables to the docker run

Towards #26, we add the ability to specify environment variables such as GITLAB_OMNIBUS_CONFIG via a new Gitlab::QA::Docker::Gitlab#environment accessor, which accepts a hash of environment variables.

May address all or some of the need expressed in #72

For an example of this feature in use see this scratch commit: https://gitlab.com/richardc/gitlab-qa/commit/00d1464803e727e309d7bcc2ab3deb747b6ca42b

mentioned in merge request !76 (closed)

unmarked as a Work In Progress

changed title from WIP: add ability to specify environment variables to the docker run to Add ability to specify environment variables to the docker run

changed the description

assigned to @grzesiek

@richardc Nice work! I left a few notes.

assigned to @richardc

marked as a Work In Progress from richardc/gitlab-qa@77de9a3e2ec1ec35d8be56dc2ff35251a283dd50

mentioned in commit 5ae7a5a7

changed the description