feat(dockerfile) support multiple files

[dduportal]

This PR adds the support of multiple files for the resources of kind dockerfile (conditions and targets).

Use cases:

• It is an additional feature to help for Feature Request: Support multiple files with spec: files in conditions of kind:file #1026.
• Also, on the Jenkins project, we have cases were we want to update the same instruction to multiple files: https://github.com/jenkinsci/docker-agent/blob/1182cad9d14ece000ab38433eee05daf1872a993/updatecli/updatecli.d/remoting.yaml#L37-L81

Test

To test this pull request, you can run the following commands:

cd pkg/plugins/resources/dockerfile
go test

Please note that a new e2e test as added for dockerfile which tests the current file (for non regression) and the new files.

I've tested this E2E test locally with a diff and an apply. The only untested part is when using with an SCM.

Additional Information

Tradeoff

Using autodiscovery would help, of course but we want to control the list of files for now.

Potential improvement

N.A.

[olblak]

Great pulllrequest, I just did some testing locally and it works great 🎆

[dduportal]

Associated documentation PR: updatecli/website#1207