docker: add labels metadata

[v1v]

Added some labels to easily identify what's the agent version that the opbeans is consuming, in addition to some labels that are generated for some other Elastic docker images.

Tests

IMAGE=docker.elastic.co/observability-ci/opbeans-python:63c87118dd931fdcf93bc7a6a26fb36b69546782
$ docker pull ${IMAGE}
$ docker inspect ${IMAGE} | jq -r '.[0].Config.Labels'
{
  "org.label-schema.license": "MIT",
  "org.label-schema.name": "opbeans-python",
  "org.label-schema.schema-version": "1.0",
  "org.label-schema.url": "https://hub.docker.com/r/opbeans/opbeans-python",
  "org.label-schema.vcs-url": "https://github.com/elastic/opbeans-python",
  "org.label-schema.vendor": "Elastic",
  "org.label-schema.version": "5.7.0"
}

[beniwohli]

LGTM

(oops, missed that this is still a draft, sorry about that)

[beniwohli]

We just released 5.8.0 yesterday, so would probably make sense to reflect that here

[v1v]

It should be bumped automatically, I'll review the process and bump it in another PR with the agent dependency, does it sound good?

[apmmachine]

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• Build Cause: [Pull request #47 opened]

• Start Time: 2020-07-01T11:57:21.009+0000

• Duration: 9 min 54 sec

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Total	5