WIP: Install latest git and git-lfs for QA docker image

What does this MR do?

This MR installs the latest versions of git (2.23.0) and git-lfs (2.8.0) into the QA Docker image. It allows easy upgrading by simply updating the GIT_VERSION or GIT_LFS_VERSION environment variables.

Related gitlab-qa#402

Proof

Using ashmckenzie/gitlab-qa as the local Docker image name built using the updated Dockerfile:

$ docker run --rm -ti --entrypoint bash ashmckenzie/gitlab-qa
root@5bde7ccbd6a9:/home/qa# git --version
git version 2.23.0
root@5bde7ccbd6a9:/home/qa# git-lfs --version
git-lfs/2.8.0 (GitHub; linux arm64; go 1.12.7; git 30af66bb)

Docker image size comparison

Previously:

$ docker image ls gitlab/gitlab-ee-qa:nightly
REPOSITORY            TAG                 IMAGE ID            CREATED             SIZE
gitlab/gitlab-ee-qa   nightly             0eb216ab29b1        13 minutes ago      1.83GB

With this MR:

$ docker image ls ashmckenzie/gitlab-qa
REPOSITORY                  TAG                 IMAGE ID            CREATED              SIZE
ashmckenzie/gitlab-qa       latest              508dc936458c        About a minute ago   1.96GB

Does this MR meet the acceptance criteria?

Conformity

• [ ] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [ ] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [ ] Database guides
• [ ] Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team