Remove (theme)Dockerfile (just-the-docs#1299)

It doesn't seem like any of our theme developers use this (out of date) `Dockerfile`, and we're probably better served placing more of this in the theme template (so users can consume it). I think this is part of our overall movement to not have users of our theme directly clone this repo.

README.md

@@ -32,7 +32,7 @@ More specifically, the created site:
 
 Other than that, you're free to customize sites that you create with the template, however you like. You can easily change the versions of `just-the-docs` and Jekyll it uses, as well as adding further plugins.
 
-### Use RubyGems
+### Use as a Ruby Gem
 
 Alternatively, you can install the theme as a Ruby Gem, without creating a new site.
 
@@ -48,17 +48,11 @@ And add this line to your Jekyll site's `_config.yml`:
 theme: just-the-docs
 ```
 
-And then execute:
+And then install all relevant dependencies:
 
-    $ bundle
-
-Or install it yourself as:
-
-    $ gem install just-the-docs
-
-Alternatively, you can run it inside Docker while developing your site
-
-    $ docker-compose up
+```shell
+$ bundle
+```
 
 ## Usage