Add Docker build type

[jasontedor]

This commit adds a new build type (together with deb/rpm/tar/zip) to represent the official Docker images. This build type will be displayed in APIs such as the main and nodes info APIs.

[elasticmachine]

Pinging @elastic/es-core-infra

[jasontedor]

With this we now have:

curl -XGET localhost:9200/
{
  "name" : "aff818783063",
  "cluster_name" : "docker-cluster",
  "cluster_uuid" : "AZ35nHzdSL6wVAI8eBuT8Q",
  "version" : {
    "number" : "8.0.0-SNAPSHOT",
    "build_flavor" : "default",
    "build_type" : "docker",
    "build_hash" : "abf4c38",
    "build_date" : "2019-02-25T21:36:00.160912Z",
    "build_snapshot" : true,
    "lucene_version" : "8.0.0",
    "minimum_wire_compatibility_version" : "7.1.0",
    "minimum_index_compatibility_version" : "7.0.0"
  },
  "tagline" : "You Know, for Search"
}

Note the build_type. Previously this would say tar for the Docker images.

[alpar-t]

This LGTM, but I would love to have a test for it, that sed command feels a bit fragile on the long run. We don't set up tests yet, but we do bring the appliance on and wait for it to start listening on TCP ports, maybe we could add this check directly to the build script and move it to a more appropriate test once we add the test infrastructure.

[jasontedor]

I would rather that prioritize having real test infrastructure here: #37617

[alpar-t]

The test infrastructure is added in #39515. I'm OK to merge this if there's any urgency and add the test once that's merged.