fusermount is not installed in keybaseio/client docker image

I am trying to use the `keybaseio/client:stable` docker image, according to this page: https://github.com/keybase/client/tree/master/packaging/linux/docker

When I use this option (-e KEYBASE_KBFS_ARGS="-mount-type=default") to mount KBFS, I get the following errror:

ERROR Mounting the filesystem failed: fusermount: exec: "fusermount": executable file not found in $PATH

It looks like `fusermount` just isn't installed. Can this be added to the image?

Start with the packaging/linux/docker configuration linked in the issue and inspect how keybaseio/client:stable is built. Verify whether fusermount is available in the resulting image when using KEYBASE_KBFS_ARGS="-mount-type=default"; done means the reported missing-executable error no longer occurs.

Written by the indexing model from the issue text.