[MRG] Hide `apt-get update` output

[yuvipanda]

apt-get update produces a lot of output that's mostly irrelevant unless there is an error. This PR will suppress stdout, but stderr will still show - so we can see errors if they happen.

[manics]

Is there some other Docker/apt config hiding somewhere? When I run apt-get -qq update interactively I don't get any output, whereas I do with apt-get update:

root@385cd7180d59:/# apt-get -qq update
root@385cd7180d59:/# apt-get update
Hit:1 http://archive.ubuntu.com/ubuntu bionic InRelease
Hit:2 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
Hit:3 http://archive.ubuntu.com/ubuntu bionic-backports InRelease              
Hit:4 http://security.ubuntu.com/ubuntu bionic-security InRelease              
Reading package lists... Done                               

[consideRatio]

I verified what @manics described, that with -qq as we already have, adding > /dev/null doesn't reduce something further - the output is blank.

(base) ➜  repo2docker-service git:(main) sudo apt-get -qq update
[sudo] password for erik: 
(base) ➜  repo2docker-service git:(main)

Closing this to tidy up our list of open PRs!