workflows: fix docker tag

[dawidd6]

https://github.com/Homebrew/brew/runs/595913492
#7379

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

[sjackman]

The first part homebrew of homebrew_sut is the name of the current working directory, translated to lower case. This PR implies that the current working directory is not /home/linuxbrew/.linuxbrew/Homebrew. Should it be?

[sjackman]

Also, thanks for the fix, Dawid!