Dependencies issues and fixes

[metonymic-smokey]

@LeoVerto some dependency fixes that need to be added:

1. Mac users need to run an additional brew install graphviz after the instructions in the README to install Graphviz dependencies and add them to their path.
   if not, this is the error faced:

graphviz.backend.ExecutableNotFound: failed to execute ['sfdp', '-Tsvg', '-O', 'graph'], make sure the Graphviz executables are on your systems' PATH

2. This tool works with docker version 2.1.0. Hence, if using pip3 to install it, use pip3 install docker==2.1.0.
   The latest version of docker (4.1.x) is not compatible with some parts of the code.

Thanks!

[LeoVerto]

Thank you for reporting this! At least for me 3.x versions of docker work so I've pinned the project to that for now.

I'll have to get around to adding instructions for installing graphviz for different operating systems.