Use iron/gcc:dev for building your C/C++ programs and iron/gcc for running them.

docker build -t iron/gcc:latest .

NOTE: Since this doesn't need any C specific stuff, use the versions from the dev build.

Tag the version, check it with docker run --rm iron/gcc:dev gcc -v:

docker tag -f iron/gcc:latest iron/gcc:5
docker tag -f iron/gcc:latest iron/gcc:5.Y
docker tag -f iron/gcc:latest iron/gcc:5.Y.Z

Push:

docker push iron/go