Add a command for building the artifacts inside Docker

[tsg]

This adds:

• A Dockerfile.artifacts with the dependencies required to run the
  rake artifact:all command inside Docker
• A ./ci/docker_build_artifacts.sh script that makes use of this image
• A gradle task that simply executes that script

[tsg]

I opened this PR to have a place to discuss it and to have the Dockefile pushed somewhere, but TBH I find the value of this somewhat dubious myself:

• Gradle already has some tasks that build artifacts natively, but not the deb and rpm. Perhaps native gradle tasks, avoiding rake altogether is the better way to go.
• Doing this inside docker, at least with this volume binding approach, is slow (known issue with rake and Docker). It takes up to half an hour on my machine.
• The rake artifact:all target downloads gradle & jruby and others. These could be instead installed in the docker image (and therefore cached between builds), but I didn't want to risk breaking the release-manager.
• I wished I could have used the logstash base image, but the JVM from it still segfaults for me (another issue with rake, I guess).

I'm opened to suggestions or just dropping this PR until we figure out something better.

[original-brownbear]

@tsg

deb and rpm. Perhaps native gradle tasks, avoiding rake altogether is the better way to go.

I'm not aware of any portable way of building rpms and debs that we could use here. The rake tasks for rpm and deb use external dependencies and those we'd still have when building via Gradle directly.
=> imo having a Dockerized version of this is kinda nice :)

[andrewvc]

I love this PR, but I'm worried about dealing with, and updating, two different base images.

Could we give the gradle thing a shot, and fall back to this strategy if that fails?

[andrewvc]

I think the reason this breaks on docker is that we don't have a gradle task for it. If we had ./gradlew artifactAll invoke the rake task it would probably not segfault. Here's an example gradle task that invokes rake https://github.com/elastic/logstash/blob/master/build.gradle#L176

[tsg]

Indeed, that does the trick, I'll update the PR.