Use of mamba is deprecated

The [`Dockerfile`](https://github.com/snakemake/snakemake-tutorial-data/blob/9b36a37ca5463ee7b05d205def54b534697f8523/Dockerfile) still uses `mamba` to set up the environment while it's been deprecated long time ago (see https://github.com/snakemake/snakemake/pull/3121) in favour of `conda`.