Task 1723: improved deployment

[LourensVeen]

This makes some improvements needed to be able to run Mahiru in a more realistic, distributed setting.

When running behind a reverse proxy, the outside API endpoint address doesn't match what we're listening on directly. In order to be able to generate correct redirects and URL references, we need to know where the user thinks we are. 63620c0 adds code to get that from the environment, which can be set appropriately by the reverse proxy.

For debugging purposes it's nice if you can set the log level, so ec36135 adds that and configures logging to output to standard out, which is nice when you're running inside of a container because the output will end up being available via docker logs <container>

Finally, compute containers need to know where the asset containers are that they get their inputs from and write their outputs to. That was done via a configuration file bind-mounted into the container, but that's inconvenient when the container is going to be run on a different machine. Since it's not a lot of data, 0c90910 just puts the JSON inside an environment variable instead.

[LourensVeen]

One more step to implementing #61.