Include envsubst for kustomize plugin use

Feature description

Requesting that the envsubst binary be added to this image.

Use cases

As kustomize does not support templating, a common use case is to pass its output through sed or envsubst to replace runtime values (such as dynamically generated hostnames)

Benefits

Sed replacements do not scale very well as the number of replacements grow. Envsubst has a more manageable interface.

Envsubst via the drone go module is also how flux2 recommends replicating their controller post-build substitutions, so this will add parity to the functionality in flux2.

Requirements

• envsubst in PATH either via the gettext RPM package or drone go binary

Links / references

• https://repo1.dso.mil/platform-one/private/big-bang/archaeopteryx-bootstrap/-/blob/master/apps/argocd/base/overlays/argocd-cm.yaml#L37
• https://fluxcd.io/docs/components/kustomize/kustomization/#variable-substitution
• https://github.com/drone/envsubst

Definition of Done

• Feature has been implemented

/cc @ironbank-notifications/feature