Refactor integration tests entrypoint scripts

[tb06904]

A few issues with the current kerberos entrypoint scripts have been identified under an existing PR these should be addressed to improve the robustness and decrease technical debt.

Originally posted by @GCHQDeveloper314 in #335 (comment)

[GCHQDeveloper314]

The issues also exist for the non-Kerberos integration test scripts, which the Kerberos ones are derived from. There's also a general scripting style few fixes which could apply to other dockerfiles.