`dot` missing from Docker image

[Carlo Alberto Ferraris]

The clever/godoc-docker image is lacking graphviz/dot, so when the "graph" link is clicked you get an internal server error, and this line in the logs:

web_1    | 2020/01/15 01:43:04 Error serving /github.com/something/something?import-graph: exec: "dot": executable file not found in $PATH