Support updating variables inside of a loop

[sudo-bmitch]

Background: While working through some nginx-proxy issues, I found that it doesn't behave nicely if a container publishing the trigger variables isn't connected to any common networks with the proxy. I could solve this with a template update, but that update needs to get and set a variable inside of a loop that can be read outside (to open the location on the the first match in the loop, and to close the location block only if it was opened by a match inside the loop).

Feature request: Modifying a variable isn't supported in the go template language (you can replace it, but only in your current scope, which doesn't help with a loop), so helper functions are needed. Hugo has implemented something very similar with Scratch to solve this problem: https://gohugo.io/extras/scratch/. Adding similar functions to docker-gen would be useful to support more complex templates. Further background on the go template limitation can be seen in this issue: golang/go#10608

(Note, this is only a wishlist for me. I've worked around my original issue by making a unique set of trigger variables per nginx-proxy in my environment.)

[rhansen]

You can declare a variable outside the loop and update it inside the loop:

{{ $x := "" }}
{{ range $i := .Foo }}{{ $x = $i }}{{ end }}
{{ $x }}

[sudo-bmitch]

Looks like in the years since I opened this, the upstream Go issue was resolved.