Fix docker image build

[federicaagostini]

Last successful docker build was more than one year ago. Then it was failing with

#9 111.3   Downloading https://files.pythonhosted.org/packages/da/f6/c83229dcc3635cdeb51874184241a9508ada15d8baa337a41093fab58011/pip-21.3.1.tar.gz (1.7MB)
#9 111.9     Complete output from command python setup.py egg_info:
#9 111.9     Traceback (most recent call last):
#9 111.9       File "<string>", line 1, in <module>
#9 111.9       File "/tmp/pip-build-EQRZKX/pip/setup.py", line 7
#9 111.9         def read(rel_path: str) -> str:
#9 111.9                          ^
#9 111.9     SyntaxError: invalid syntax
#9 111.9     
#9 111.9     ----------------------------------------
#9 112.0 Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-build-EQRZKX/pip/
#9 112.1 You are using pip version 8.1.2, however version 21.3.1 is available.
#9 112.1 You should consider upgrading via the 'pip install --upgrade pip' command.

Switching to python3 solved the issue. The docker image now is the same as for escape-auth-tests.

Since the CI is triggered at any push on the master branch, I've tested the code locally:

• built docker image with tag andreaceccanti/wlcg-jwt-compliance-tests
• run test suite (same tag for the image is used in the docker-compose)

and it works fine.