feat: add WithDefault variants to groupBy template functions

[buchdag]

The PR add two new template function:

• groupByWithDefault $containers $fieldPath $defaultValue
• groupByLabelWithDefault $containers $label $defaultValue.

They both work as their counterpart groupBy and groupByLabel, but do not omit the containers that do not have a value for the provided field path expression or label, and instead group those under the provided $defaultValue.

I'm less convinced of the usefulness of a groupByMultiWithDefault template function, but maybe I'm wrong.

I'm not certain how a groupByKeysWithDefault $containers $fieldPath $defaultValue should work to be most useful and consistent:

• return an array containing only the $defaultValue when no containers have the field path set ?
• return an array containing the $defaultValue for every container that don't have the field path set ?

ping @pini-gh

[pini-gh]

I'm less convinced of the usefulness of a groupByMultiWithDefault template function, but maybe I'm wrong.

It's not needed for nginx-proxy, but could be for other use cases.

I'm not certain how a groupByKeysWithDefault $containers $fieldPath $defaultValue should work to be most useful and consistent:

• return an array containing the $defaultValue when at least one container doesn't have the field path set

[buchdag]

This solution and the second one I proposed might cause issues in cases where we do stuff like this:

{{- $foot := or (first (groupByKeys $containers "Env.FOO")) "" }}

If we replace it with:

{{- $foot := first (groupByKeysWithDefault $containers "Env.FOO" "") }}

If the first container iterated over doesn't have the field path set, $foo is assigned the default value despite other containers having a value set for Env.FOO

Maybe we don't really need groupByKeysWithDefault right now because it does not allow us to substantially simplify the nginx-proxy template ?

[pini-gh]

If the first container iterated over doesn't have the field path set, $foo is assigned the default value despite other containers having a value set for Env.FOO

Yes, indeed. But it makes no sense to use groupByKeysWithDefault when the purpose is to get the first non-default value.

[buchdag]

If that's okay with you I'll merge this PR as is and release a new version of docker-gen, then come back later for groupByMultiWithDefault and groupByKeysWithDefault.

In the context of nginx-proxy's template, I'm not really convinced groupByLabelWithDefault will immediately help us either because we're using groupByLabel kind of for the same purpose as groupByKeys (get the first set value of a given label or field path among a list of containers).

[pini-gh]

Yes that's fine.