add D-in-D dockerfile, update Readme

Dockerfile.dind

@@ -0,0 +1,33 @@
+# Use the official Docker docker-in-docker Alpine image as the base image
+FROM docker:dind
+
+# Set the working directory to /mrsk
+WORKDIR /mrsk
+
+# Copy the Gemfile, Gemfile.lock into the container
+COPY Gemfile Gemfile.lock mrsk.gemspec ./
+
+# Required in mrsk.gemspec
+COPY lib/mrsk/version.rb /mrsk/lib/mrsk/version.rb
+
+# Install system dependencies including ruby-dev
+RUN apk add --no-cache --update ruby-dev build-base git docker openrc \
+    && rc-update add docker boot \
+    && gem install bundler --version=2.4.3 \
+    && bundle install
+
+# Copy the rest of our application code into the container.
+# We do this after bundle install, to avoid having to run bundle
+# everytime we do small fixes in the source code.
+COPY . .
+
+# Install the gem locally from the project folder
+RUN gem build mrsk.gemspec && \
+    gem install ./mrsk-*.gem --no-document
+
+# Set the working directory to /workdir
+WORKDIR /workdir
+
+# Set the entrypoint to run the installed binary in /workdir
+# Example:  docker run -it -v "$PWD:/workdir" mrsk init
+ENTRYPOINT ["mrsk"]