Added RuboCop.

README.md

@@ -1,9 +1,13 @@
 # GitHub actions
 
-Some GitHub actions that I use in my projects.
+A collection of GitHub actions that I use in my projects.
 
 ## Actions
 
-### eslint
+### ESLint
 
-JavaScript linter.
+ESLint is a JavaScript linter.
+
+### RuboCop
+
+RuboCop is a Ruby static code analyzer and code formatter.

rubocop/Dockerfile

@@ -0,0 +1,20 @@
+FROM ruby:2.6.0-alpine3.8@sha256:2b4d28f53aa285e5566963c5592a365bfbdd7b38a8f53550ca1ef5fe00b10fd4
+
+LABEL repository="https://github.com/koenrh/rubocop-action"
+LABEL maintainer="Koen Rouwhorst <info@koenrouwhorst.nl>"
+
+LABEL "com.github.actions.name"="RuboCop"
+LABEL "com.github.actions.description"="RuboCop is a Ruby static code analyzer and code formatter."
+LABEL "com.github.actions.icon"="flag"
+LABEL "com.github.actions.color"="red"
+
+RUN apk add --no-cache \
+  build-base=~0.5
+
+RUN addgroup -S rubocop && \
+  adduser -S -g rubocop rubocop
+
+RUN gem install rubocop --no-document
+
+COPY entrypoint.sh /entrypoint.sh
+ENTRYPOINT ["/entrypoint.sh"]

rubocop/entrypoint.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+set -e
+
+sh -c "rubocop $*"