.gitlab-ci/Dockerfile: fix CI
as of June 10th 2023, debian:stable points to Debian 12 which doesn't have a python package, we don't need it since all supported Turris OS versions only use Python 3
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as of June 10th 2023, debian:stable points to Debian 12 which doesn't have a python package, we don't need it since all supported Turris OS versions only use Python 3