Allow os.Setenv to pass through to docker containers

[kegsay]

So authors can do stuff like DENDRITE_TRACE_INTERNAL=1

[reivilibre]

I'm not sure I understand the title of this issue (os.Setenv seems to be a function for setting environment vars?).

However I would also like the ability to pass environment variables through to Synapse's container.
Do we just pass all the variables in, or should we be a bit more cautious (a typical process/shell usually has a lot of ambient environment variables; I don't want to interfere with the reproducibility too much)?
As a first thought: if we care about that, they could be prefixed with a common prefix — not sure what your preference is there?