To build a gcc using the docker image:
docker run -v $(pwd):/out compilerexplorer/gcc-builder bash build.sh 6.2.0 /out/gcc-6.2.0.tar.gz
Note you'll need lots of disk space (32GB+).
If like me, you want to run on a RancherOS EC2 image, you'll need to do something like:
docker run --privileged -i --rm ubuntu bash << EOF
apt-get update
apt-get install -y cloud-guest-utils parted
growpart /dev/xvda 1
partprobe
resize2fs /dev/xvda1
EOF
...to ensure the RancherOS partition uses all available space.