Add proxy environment to allow install packages with yum from conan

conan-el7-gcc48/docker-compose.yml

conan-el7-gcc73-x86/Dockerfile

conan-el7-gcc73-x86/docker-compose.yml

conan-el7-gcc73/Dockerfile

conan-el7-gcc73/docker-compose.yml

conan-el7-gcc82-x86/Dockerfile

conan-el7-gcc82-x86/docker-compose.yml

conan-el7-gcc83/Dockerfile

conan-el7-gcc83/docker-compose.yml

conan-el7-gcc93-x86/Dockerfile

conan-el7-gcc93-x86/docker-compose.yml

conan-el7-gcc93/Dockerfile

conan-el7-gcc93/docker-compose.yml

environment