Pin localstack to 0.12.8

[rnorth]

It seems that our build is afflicted by localstack/localstack#3816
Localstack builds >=0.12.9 seem to have this issue, so pinning our test dependency to 0.12.8 for now.

[rnorth]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[rnorth]

Added a commit to use DockerClient to tag images rather than execing docker tag. As part of this I've also tidied up the test a little.

Note that I think we can further tidy this test - we should really aim to cut a lot of the execution time down by testing the LocalStackContainer.shouldRunInLegacyMode method directly, rather than having an integration test for so many different permutations. I think that should be done separately, though.