This PR adds support for docker-compose.yml files in the docker package. The approach was to add additional handlers alongside docker/lib/dependabot/docker, i.e. docker/lib/dependabot/docker_compose.

To address the duplication of code resulting from identical logic in e.g. parsing Docker image names or computing resulting updates some shared logic was extracted to modules under docker/lib/dependabot/docker/utils, which then get pulled in as mixins. For tests, I've added e.g. docker/spec/dependabot/docker/common/shared_examples_for_docker_update_checkers.rb, since most of the code is exactly the same.

Suggestions are welcome, especially on:

• the correct location for the docker_compose folder;
• the best way to share common Docker logic between docker and docker_compose;
• whether naming this as docker_compose is the right way. I see that some languages already support multiple package managers in Dependabot (e.g. go:dep and go:modules), so I'm wondering if this should be named docker:compose in this list. I'm trying to figure out where that is configured, just searching for go:modules doesn't yield results.

Closes #390

[dependabot-core-dev] ~/dependabot-core $ bin/dry-run.rb docker_compose buildkite/agent --dir .buildkite
warning: parser/current is loading parser/ruby26, which recognizes
warning: 2.6.5-compliant syntax, but you are running 2.6.2.
warning: please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.
=> fetching dependency files
=> dumping fetched dependency files: ./dry-run/buildkite/agent/.buildkite
=> parsing dependency files
=> updating 1 dependencies

=== golang (1.12)
 => checking for updates
 => latest version from registry is 1.13.1
 => latest resolvable version is 1.13.1
 => requirements to unlock: own
 => requirements update strategy: 
 => updating golang from 1.12 to 1.13.1

    ± docker-compose.yml
    ~~~
    5c5
    <     image: golang:1.12
    ---
    >     image: golang:1.13.1
    ~~~