build_docker.sh: push to GCR repo in addition to dockerhub repo (#3817)
build_docker.sh: push to GCR repo in addition to dockerhub repo (#3817)

Resolves #3793
frank-y-liu/gatk,ksthesis/gatk,ksthesis/gatk,frank-y-liu/gatk,magicDGS/gatk,ksthesis/gatk,broadinstitute/hellbender,broadinstitute/hellbender,magicDGS/gatk,magicDGS/gatk,frank-y-liu/gatk,ksthesis/gatk,ksthesis/gatk,broadinstitute/hellbender,broadinstitute/hellbender,frank-y-liu/gatk,magicDGS/gatk,broadinstitute/hellbender,broadinstitute/hellbender,frank-y-liu/gatk,magicDGS/gatk,frank-y-liu/gatk,ksthesis/gatk,magicDGS/gatk