Add support for docker-compose.yml files

[pedropombeiro]

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
    ~~~

[toolmantim]

Thanks @pombeirp! Would love to see support for docker-compose.yml land ❤️

[pedropombeiro]

@feelepxyz Don't know what is happening but the tests show as failing in the PR (due to stuff unrelated to the PR).

[rebelagentm]

@pombeirp @feelepxyz It looks like the tests are passing again.

[feelepxyz]

@pombeirp thanks for the contribution! Just a heads up that I won't be able to get to this immediately but hopefully get some time in the next week or so to review it thoroughly to make sure we understand the changes and can maintain it going forward.

[pedropombeiro]

@feelepxyz Sure, totally understand it, thanks for the transparency.

[badgerwithagun]

Looking forward to this

[ajcann]

Also looking forward to this getting in!

[erkannt]

@pedropombeiro Can you tell us why this PR was closed? Would love to use this.

[pedropombeiro]

@erkannt There was no feedback from reviewers for about 4 months, and because I keep a widget of open PRs in VS Code, I decided to clean that up a bit.

[falkenhawk]

@erkannt There was no feedback from reviewers for about 4 months, and because I keep a widget of open PRs in VS Code, I decided to clean that up a bit.

😭

[lsh-0]

Think we can re-open this? Who do we have to prod to get some review feedback?

[bobvanderlinden]

@rebelagentm could you have a look again?