Running a Docker Container with GUI on Mac OS
Got the idea/big picture from here: https://sourabhbajaj.com/blog/2017/02/07/gui-applications-docker-mac/, but had to do some more steps:
1. Install XQuartz
brew install xquartz2. Run XQuartz
open -a XQuartz3. Allow Network Connections (XQuartz/Preferences):

4. Get the Host IP
IP=$(ifconfig en0 | grep inet | awk '$1=="inet" {print $2}')5. SET DISPLAY Environment Variable
export DISPLAY=$IP:06. Add Path to "xhost" to my zsh-Profile; added this line to .zshrc:
export PATH=/usr/X11/bin/xhost:$PATH7. Added the IP with xhost
xhost + $IP8. Run Firefox from Docker Container
docker run -d --name firefox -e DISPLAY=$IP:0 -v /tmp/.X11-unix:/tmp/.X11-unix jess/firefox