goinsu - a simple su which doesn't mess with TTY and other stuff. It is as simple as "Hey, run this program as this user".
$ docker run -it --rm ubuntu:trusty su -c "ps aux"
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 7.0 0.0 46644 2628 pts/0 Ss+ 19:07 0:00 su -c ps aux
root 7 0.0 0.0 15584 2100 ? Rs 19:07 0:00 ps aux
$ docker run -it --rm ubuntu:trusty sudo ps aux
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 7.0 0.0 46028 3044 pts/0 Ss+ 19:07 0:00 sudo ps aux
root 6 0.0 0.0 15584 2140 pts/0 R+ 19:07 0:00 ps aux
$ docker run -it --rm -v`pwd`/goinsu:/usr/bin/goinsu ubuntu:trusty goinsu root ps aux
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 0.0 0.0 7152 852 pts/0 Rs+ 19:08 0:00 ps aux
goinsu does the same thing but in 10KB (well, 50KB if compiled as a static executable) instead of 1.8MB.
goinsu is written in the D programming language, which is a high-level programming language with metaprogramming, CTFE, and other cool things you can't find in C.
Yep. goinsu written in a subset of the D programming language, called betterC. Just google for it, ya' know.
You can use gosu or su-exec, but they aren't as cool as goinsu.