bundle install or bundle update
bundle exec middleman serve
bundle exec middleman build

on Mac OSX The following is needed to properly execute the first line to get ffi t install (might need to repeat again without env variables after ffi installs if other packages fail.)

LDFLAGS="-L/usr/local/opt/libffi/lib" PKG_CONFIG_PATH="/usr/local/opt/libffi/lib/pkgconfig" bundle install --path vendor

Alternatively compile it using a docker container:

docker run -v "$PWD":/app nielsvdoorn/middleman bundle install && bower install && bundle exec middleman build

Or even easier, run the build the exact same way it will be run by the gitlab CI. Keep in mind this will not pull from your local directory though, it pulls from whatever is in git:

gitlab-runner exec docker pages