Draft: Allow dast container uid to be set via env

What does this MR do?

allows current user's uid to be configured in tests and scripts.

Why?

when you mount a volume with docker, the uid of the files and directories remain the same as those on the host. as a result, you may not be able to to write to those files or directories when you mount them. this issue is manifest on my local development environment which uses docker-machine and virtualbox and i've also been able to reproduce it on a fresh debian installation on a vm.

What are the relevant issue numbers?

gitlab-org/gitlab#288010 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer