pass a gitlab token from environment to build private packages

Carlo FERRIGNO requested to merge ferrigno/ci-template-cc:master into master

If in requirements.txt some packages have restricted access, it is possible to use a token in gitlab. By using the environmental variable ASTRO_GITLAB_PIP_TOKEN, I can pass it to build the container. I added two parameters to nb2workflow container module to handle this (see corresponding pull request on github).

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