refactor: drop swiftenv Docker container

As Swift now maintain official Docker images, this supersedes needing to
run swiftenv in Docker. See https://hub.docker.com/_/swift/ instead.

Author: kylef

README.md

@@ -27,6 +27,3 @@ swiftenv allows you to:
 - Integrations
     - [Heroku](https://swiftenv.fuller.li/en/latest/integrations/heroku.html)
     - [Travis CI](https://swiftenv.fuller.li/en/latest/integrations/travis-ci.html)
-    - [GitLab CI](https://swiftenv.fuller.li/en/latest/integrations/gitlab-ci.html)
-    - [CircleCI](https://swiftenv.fuller.li/en/latest/integrations/circle-ci.html)
-    - [Docker](https://swiftenv.fuller.li/en/latest/integrations/docker.html)

docs/index.rst

@@ -32,7 +32,4 @@ Integrations
 
    integrations/heroku
    integrations/travis-ci
-   integrations/gitlab-ci
-   integrations/circle-ci
-   integrations/docker
    integrations/cocoapods