Make RenkuLab run smoothly behind a web proxy

[ableuler]

Many organizations require that outgoing http(s) requests are routed through a web proxy. Requests to the organization domain are often exempt form this rule. When RenkuLab is deployed behind such a web-proxy, some outgoing requests will be blocked (depending on the implementation and configuration, those can be more or fewer). For a first step, we're going to focus on a scenario where requests to the organization domain(s) do not have to use a poxy.

To my knowledge - even in this scenario - there are at least 3 locations where https clients have to be configured to use the proxy for such a setup to work.

• renku core service (when cloning the template repo from github through https, adding proxy settings to .gitconfig should do the job)
• renku-ui (querying project template repo before project creation dialogue is shown, https://github.com/TooTallNate/node-https-proxy-agent seems to be the default for using the fetch api with a proxy).
• Any outgoing http(s) requests from within user sessions (not sure how tricky it will be to pre-configure a user session in a way that all possible clients will pick up the proxy setting, or wether that's possible at all).

[MichaelAML]

For the interactive user sessions it should be enough if you can set the corresponding environment variables, i.e. set HTTP_PROXY, HTTPS_PROXY, as well as NO_PROXY in the Docker images.