[Dockerfile] Faster runs after local changes

This segments the portions of the `docker build` from ones that are more
likely in between project file changes between test runs and ones that
don't to reduce the re-build time when running tests via docker

Context
-------

So when you first run `rake test:docker:all` on a fresh install, you
should expect this to take a bit since the docker cache isn't warmed and
the steps of the image build process need to download items.  You
probably can expect this to take the sum total of about 10ish minutes
the first time around.

Some of this will be shared between versions of ruby (apt packages,
installing `rvm`, etc.), but once you start installing specific versions
of ruby, they images will diverge.

That said, once you have gone through the build, subsequent runs with no
changes take only about 10 seconds:

    $ rake test:docker

    ...

    real    0m10.660s
    user    0m0.819s
    sys     0m0.422s

However, prior to this change, any file change made in the project dir
required rebuilding everything installing bundler on.  This ended up
increasing the time to about a minute:

    $ rake test:docker

    ...

    real    1m10.171s
    user    0m0.668s
    sys     0m0.392s

Changes
-------

With this change, we push the bundling to happen in a separate directory
from the final working dir, and only include the files necessary to
perform that action.  This decreases the time it takes to run the entire
suite across all versions by 40 seconds:

    $ rake test:docker

    ...

    real    0m26.009s
    user    0m0.858s
    sys     0m0.464s

We could further improve this by segmenting out the `rake build` steps,
but for now I think this is an acceptable speed for most, and that is a
slightly more complicated process to make that happen.  Doing so would
require not only moving the files we need just to compile done
separately, but also moving the rest of the files that aren't involved
with the C-extension separately.

Author: NickLaMuro

Dockerfile

@@ -14,9 +14,17 @@ RUN curl -sSL https://get.rvm.io | bash -s
 ARG RVM_RUBY_VERSION=ruby-head
 RUN /bin/bash -l -c "echo $RVM_RUBY_VERSION"
 RUN /bin/bash -l -c ". /etc/profile.d/rvm.sh && rvm install $RVM_RUBY_VERSION --binary || rvm install $RVM_RUBY_VERSION"
-ADD . /stackprof/
-WORKDIR /stackprof/
+
+# Gem Dependencies
 RUN /bin/bash -l -c ". /etc/profile.d/rvm.sh && gem install bundler:1.16.0"
+ADD Gemfile Gemfile.lock stackprof.gemspec /bundle/
+WORKDIR /bundle/
 RUN /bin/bash -l -c ". /etc/profile.d/rvm.sh && bundle install"
+
+# Build Extension
+ADD . /stackprof/
+WORKDIR /stackprof/
 RUN /bin/bash -l -c ". /etc/profile.d/rvm.sh && bundle exec rake build"
-CMD /bin/bash -l -c ". /etc/profile.d/rvm.sh && bundle exec rake"
+
+# Test
+CMD /bin/bash -l -c ". /etc/profile.d/rvm.sh && TESTOPTS='--seed=1355' bundle exec rake"