Use Dockerfile flow for QA Auto DevOps

What does this MR do?

Adding a Dockerfile using alpine results in 26MB images vs 400MB images which is faster to upload and download. This does mean we lose the default herokuish QA coverage but we should have adequate coverage in https://gitlab.com/gitlab-org/cluster-integration/auto-build-image/ now.

Triva: herokuish is bigger because its base image is based on ubuntu. (Credit to @tigerwnz for working this out)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] 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