Add graphviz in Dockerfile

[MaxDesiatov]

graphviz is not currently installed, which triggers warnings when running the corresponding GitHub Action.

[mattt]

Thanks for the PR, @MaxDesiatov. I would have sworn that this was already in there, but I'm probably conflating adding GraphViz as a dependency in the Homebrew formula. Anyway, glad to have this fixed, and looking forward to having this available in the forthcoming release.

[MaxDesiatov]

Thanks, I also have a prebuilt image here that I'm using in the meantime to make the action run faster, just in case you're interested https://hub.docker.com/r/maxdesiatov/swift-doc

I hope you could upload the official one in your Docker Hub namespace at some point.

[MaxDesiatov]

(This required me to create two separate Dockerfiles in my swift-doc fork, one for the prebuilt image, the other for the action that only references the prebuilt image and does nothing else)

[mattt]

@MaxDesiatov

Thanks, I also have a prebuilt image here that I'm using in the meantime to make the action run faster, just in case you're interested https://hub.docker.com/r/maxdesiatov/swift-doc

I hope you could upload the official one in your Docker Hub namespace at some point.

That's great! Let's continue this discussion on #111.