Docker tests broken

[mrbean-bremen]

With the latest PR, all of the docker tests didn't run because the setup failed with something like:

Connecting to github.com (github.com)|192.30.255.112|:443... connected.
HTTP request sent, awaiting response... 302 Found
Location: https://codeload.github.com/jmcgeheeiv/pyfakefs/zip/merge [following]
--2021-05-13 11:47:28--  https://codeload.github.com/jmcgeheeiv/pyfakefs/zip/merge
Resolving codeload.github.com (codeload.github.com)... 192.30.255.121
Connecting to codeload.github.com (codeload.github.com)|192.30.255.121|:443... connected.
HTTP request sent, awaiting response... 404 Not Found
2021-05-13 11:47:28 ERROR 404: Not Found.

The command '/bin/sh -c mkdir -p work     && wget https://github.com/$github_repo/archive/$github_branch.zip     && unzip $github_branch.zip -d work' returned a non-zero code: 8

The docker tests will be disabled until this issue is resolved.

[mrbean-bremen]

Does not happen in my own branch with the same code, only in master.
Only happens in PR builds. I have enabled GH action builds for PRs only recently - will probably just exclude the docker tests in PRs.

[mrbean-bremen]

Works as before, shall be sufficient to have docker test on pull.