Refactor dockerfile scripts

Currently we're using following way to catch failed commands in dockerfile scripts:

set -e
update-alternatives --install /usr/bin/python python /usr/bin/python3 1 || \
    { echo "Unable to set python3 as the default python"; exit 1; }
set +e

At this point set -e is redundant. We're losing original error code as every failure is shown ar error code 1. Removing "exit 1;" will not work, because then the last command would be "echo" which would return error code 0, so docker build will go further. We need to find the way to maintain original error code and error message as painless as possible.