Review and improve containerfiles

[kn0wl3dge]

Container files are not perfect.
Since we're using a lot of them, we should try to optimize used space of images.

Also, everything is running as root which is not necessary. Some work is needed to limit containers privileges to what is really necessary.

@0pendev Can you give it a try ?

[0pendev]

I've started to look at them. I might push a few changes later this week.