feat(command): Add gem

Author: boukeversteegh

README.md

@@ -45,6 +45,7 @@ dockerized <command>
     - python
     - python2
   - ruby
+    - gem
     - rake
   - rust
     - rustc

docker-compose.yml

@@ -129,6 +129,9 @@ services:
   rake:
     <<: *ruby
     entrypoint: [ "rake" ]
+  gem:
+    <<: *ruby
+    entrypoint: [ "gem" ]
   rustc:
     image: "rust:${RUST_VERSION}"
     entrypoint: [ "rustc" ]