Bump docker-api from 2.0.0 to 2.1.0

Bumps [docker-api](https://github.com/swipely/docker-api) from 2.0.0 to 2.1.0.
- [Release notes](https://github.com/swipely/docker-api/releases)
- [Commits](https://github.com/swipely/docker-api/commits/v2.1.0)

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -124,7 +124,7 @@ GEM
     debug_inspector (1.0.0)
     diff-lcs (1.4.4)
     docile (1.3.5)
-    docker-api (2.0.0)
+    docker-api (2.1.0)
       excon (>= 0.47.0)
       multi_json
     domain_name (0.5.20190701)