feat: gettext for envsubst

What does this MR do?

This MR adds the package gettext, which includes the binary envsubst.

envsubst allows for the highly useful substitution of environment variables in (for example) values.yaml files, without having to resort to subcharts, --set or however black magic is required to make that happen in Helm.

It also makes a lot of sense given the idea that this image should support (Helm) K8s deployments in CI, and in CI is where environment variables have a great usefulness and impact to automation easing.

This is something preferably added to Helm itself, as discussed here, but I don't expect that to happen soon.

Manual QA

• Test this change using a Cluster management project

Already using this for cluster management, and it's beautiful.