[AIRFLOW-5143] Fix for potentially corrupted .jar

[potiuk]

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• Passes flake8

[BasPH]

Apache Rat also provides SHA256 checksums for verifying the integrity. WDYT of using those?

[Fokko]

Tnx for picking this up @potiuk

[potiuk]

I realised I already moved the licence check to separate Dockerfile - so I fixed it now in the RIGHT place and removed it from the CI Dockerfile as it is not needed there !

[kaxil]

LGTM.

Thanks @potiuk

[potiuk]

One more pass. One of the tests failed and I realised that we've been building checklicence image needlessly in tests.