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

Building this image

docker build -t fnproject/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 fnproject/gcc:dev gcc -v:

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

Push:

docker push fnproject/go