docker-desktop: fix bind mounts

[nzbr]

The /mnt mount gets marked as shared in the systemd wrapper script, so that the bind mounts created by the docker desktop proxy are present outside the systemd namespace

See also #89 #90

@hschaeidt

[nzbr]

Can someone verify, that this patch works for people other than me
For example docker run --rm -it -v $PWD:/pwd alpine ls -la /pwd should show the contents of the current directory, while listing an empty directory without the patch

[pogopaule]

Does not work for me.

❯ docker run --rm -it -v $PWD:/pwd alpine ls -la /pwd
total 4
drwxr-xr-x    2 root     root            40 Oct 24 12:01 .
drwxr-xr-x    1 root     root          4096 Oct 24 12:01 ..

See #148