Change the devtoolset to version 8

After the python and Qt upgrade we're using devtoolset v8 in our centos7 container.

docker/build.sh

@@ -7,7 +7,7 @@ SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
 PROJECT_DIR="$( cd "${SCRIPT_DIR}/.." && pwd )"
 
 # Make sure that environment variables are set properly
-source /opt/rh/devtoolset-7/enable
+source /opt/rh/devtoolset-8/enable
 export PATH="${CURA_BUILD_ENV_PATH}/bin:${PATH}"
 export PKG_CONFIG_PATH="${CURA_BUILD_ENV_PATH}/lib/pkgconfig:${PKG_CONFIG_PATH}"