feat: add docker context module (#996)

* feat: add docker context module

Adds a simple module that checks for a Docker config file and if present, reads the `currentContext` value out and displays on the prompt with a whale.

* feat: add `only_with_compose_yml` option to docker_context

When enabled, will only show the docker context name if there's a docker-compose.yml file in the current directory.

* Update src/modules/docker_context.rs

Co-Authored-By: Thomas O'Donnell <andytom@users.noreply.github.com>

* Update src/modules/docker_context.rs

Co-Authored-By: Thomas O'Donnell <andytom@users.noreply.github.com>

* rename `only_with_compose_yml` configuration key to a more generic `only_with_files`

* Update src/modules/docker_context.rs

Co-Authored-By: Thomas O'Donnell <andytom@users.noreply.github.com>

* re-order configuration table

* Update docs/config/README.md

Co-Authored-By: Thomas O'Donnell <andytom@users.noreply.github.com>

* Update src/configs/docker_context.rs

Co-Authored-By: Thomas O'Donnell <andytom@users.noreply.github.com>

Co-authored-by: Thomas O'Donnell <andytom@users.noreply.github.com>

src/configs/mod.rs

@@ -5,6 +5,7 @@ pub mod cmd_duration;
 pub mod conda;
 pub mod crystal;
 pub mod directory;
+pub mod docker_context;
 pub mod dotnet;
 pub mod elixir;
 pub mod elm;