Auto-enable Docker buildkit

[ANogin]

• I have reviewed the OFRAK contributor guide and attest that this pull request is in accordance with it.

One sentence summary of this PR (This should go in the CHANGELOG!)
Default to forcing Docker buildkit to be enabled.

Link to Related Issue(s)
N/A; supersedes #392

Please describe the changes in your request.
Per @EdwardLarson:

Some features in the build require Docker buildkit to be enabled, and we say as much in the docs to build the image. However, this is a point of failure (for some reason I could not get the buildkit to be enabled in my machine) and a "gotcha" that can be removed entirely by just setting the Docker buildkit enabled in the subprocess we spin up to do the Docker build. In case there is a desire to specifically NOT use the buildkit, an option is added to not alter the subprocess environment at all (buildkit may or may not be enabled depending on user's environment).

Anyone you think should look at this, specifically?
@whyitfor