gitlab-ruby: add exec-env script

What does this MR do?

Add /scripts/exec-env which will find any/all files directly under /etc/gitlab/environment and export them as environment variables before execing the final process.

1. It will only operate within /etc/gitlab/environment directory.
2. It will search /etc/gitlab/environment for files only, depth 1.
3. It will only export them if they have content (size in bytes > 0).

Related issues

Relates to gitlab-org/charts/gitlab#2725 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated

Closes #2725