Bug 1657199: Generate the same docker-image digests locally as in automation; r=Callek a=release

In bug 1626058, I changed how the docker image digest was generated:

• I used the same directory structure to generate the digest as was used for generating the context
• I moved context generation to the decision task, and used the hash of that as part of the digest.

Unfortunately, it turns out the file name in the gzip header of the context
.tar.gz differed between when we are creating a context to write out, and when
were just generating the hash.

This adjust the name used in the gzip header to be consistent.

Differential Revision: https://phabricator.services.mozilla.com/D84753