Dockerfile parse error line 5: unknown instruction: -ESSENTIAL
I assume you have a copy-paste issue. The right package name is build-essential. And if you want to split a RUN statement over several lines, you will have to use backslash for line continuation.

All that leading to something along the lines of:

RUN apt-get update -yqq; apt-get -yqq install ruby ruby-dev \
          build-essential redis-tools


As a personal suggestion, when you have several commands to run, it is usually better to join them using &amp;&amp; rather than ; since you want the build to fail if either command is failing.

RUN apt-get update -yqq &amp;&amp; apt-get -yqq install ruby ruby-dev \
          build-essential redis-tools