Docs need to specify ENV var to detect CI provider

[tomchipchase]

I've just had to add BUILDKITE to the list of environment variables passed down to docker for running CI after upgrading to 5.1.0.

It looks like this is necessary after this change #201, but the docs don't specify BUILDKITE as one of the variables we need to pass down.

[ArturT]

@tomchipchase Thanks for reporting this. I updated the docs.