Add support for Helmfile

[mikelorant]

Implemented helmfile support. Improvements have been added to the original work from https://github.com/QuoineFinancial/concourse-helmfile-resource.

Extra functionality or fixes:

• Add support for helmfile.d with auto detection.
• Helmfile command is settable.
• Upgraded helmfile version.
• Params under helmfile key.
• Prefixed helmfile specific variables for clarity.
• Fixed early execution of helmfile.

Other improvements unrelated to Helmfile support:

• Improved Dockerfile to add plugins before file assets to speed up build.
• Switch back to alpine Helm release and downgrade for better compatibility.
• Fixed references to helm when they should be using the helm_bin variable.

Have concerns about the meaning of the result as it is specific to a helm release and a helmfile can have many releases in it. Have not changed this logic and it continues to work as originally designed.

[msiegenthaler]

Hey, thanks so much!
I really do like the helmfile support but I'm a bit unsure if it makes sense to have both things in the same resource of if we'd be better of splitting it into two resources: one for helmfile support and another one for plain helm. I mean apart from the kubernetes-cluster connection they don't have that much in common anymore.

What are your thoughts about that?

[mikelorant]

I agree that in some ways it should be split, but there was so much in common it was easy to add without too much extra code. Also, Helmfile with Concourse is a much better solution than directly calling Helm. Much of the Concourse configuration can be simplified into a simpler helmfile.

Something for you to ponder, maybe others may share feedback.

[msiegenthaler]

Is there a specific reason why you want to downgrade?

[mikelorant]

No, but the image location should go back to alpine/helm. Version is painful before can't use new Helm clients with older server versions. Can't really win for that unfortunately and would make sense to go to the latest Helm 2 release.