Make conda-docker a dependency in Linux only in pyproject.toml

[asmeurer]

Fixes #578 .

Note, we will also need to make sure to fix this in the conda-forge feedstock.

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[asmeurer]

Note that conda-docker is actually perfectly pip installable on all platforms, it just won't work when the platform isn't Linux (but it can be imported).

[asmeurer]

The reason is that the Linux-only dependency, fakechroot, is not a Python package. It's an executable that gets called at runtime. It is, however, a dependency in the conda package which cannot be installed on Linux.

[trallard]

Thank you, this LGTM 🚀 merging now!