Add instructions for running locally using docker

Author: dirkcuys

.bundler/keep

@@ -1,3 +1,4 @@
 _site/*
 .idea/
 .sass-cache
+.bundler/

.gitignore

@@ -7,6 +7,11 @@ To make changes to the template itself, a good place to start is the [`_layouts`
 
 Questions? Ask on P2PU's [Community Forum](https://community.p2pu.org/c/tech/course-in-a-box/78).
 
+# Running locally
+- install docker
+- create a directory called .bundler
+- Run `docker run -i -t --rm -u 1000:1000 -p 4000:4000 -v `pwd`:/opt/app -v `pwd`/.bundler/:/opt/bundler -e BUNDLE_PATH=~/opt/bundler -w /opt/app ruby:2.7 bash -c "bundle install && bundle exec jekyll serve --watch -H 0.0.0.0"`
+
 ---
 Course-in-a-Box was built by [Peer 2 Peer University](https://www.p2pu.org) using the [P2PU Jekyll course template](https://github.com/p2pu/jekyll-course-template) and shared under an MIT License.