Implement option to disable cache rebuild if it already exists in storage

[lafriks]

Description

Adds override option that is true by default to preserve backward compatibility

Fixes #83

Checklist

• Read the CONTRIBUTING document. (It's checked since you are already here.)
• Read the CODE OF CONDUCT document.
• Add tests to cover changes.
• Ensure your code follows the code style of this project.
• Ensure CI and all other PR checks are green OR
  • Code compiles correctly.
  • Created tests which fail without the change (if possible).
  • All new and existing tests passed.
• Add your changes to Unreleased section of CHANGELOG.
• Improve and update the README (if necessary).
• Ensure documentation is up-to-date. The same file will be updated in plugin index when your PR is accepted, so it will be available for end-users at http://plugins.drone.io.

[lafriks]

I don't think test failure is related to this PR

[lafriks]

Ok, might be just tests are not stable, after force push now they succeeded

[kakkoyun]

@lafriks CI failures are unrelated, I've opened an issue about it #104.

Checking the rest of the PR now.

[kakkoyun]

Overall looks good to me. Thanks a lot for your contribution. I believe this could improve the user experience a lot. We can consider having this check by default for upcoming releases.

I have a couple of nits, it'd be awesome if you can handle those.
Also, we have a checklist https://github.com/meltwater/drone-cache/blob/master/CONTRIBUTING.md#pull-request-checklist, it'd glad if you have a look at it.

Since we add a new option we should update the CHANGELOG, I'd also forgotten to update it, just did. So rebase before you make a change on it.

[lafriks]

@kakkoyun done

[kakkoyun]

lgtm

[kakkoyun]

This will be in the next stable release v1.1.0, it shouldn't take long. I'm planning to wrap things up and tag a release candidate this weekend. Until then you can pick up a master build from https://hub.docker.com/r/meltwater/drone-cache/tags?page=1&ordering=last_updated to use in your pipeline.

[lafriks]

Could you restart drone master build task? :)

[kakkoyun]

CI is killing me 😓