Add envsubst package to docker image

[stevehipwell]

This PR adds the envsubst command to the Docker image to enable automation users to work around the limitations (by design) of kustomize.

This resolves #3138.

[stevehipwell]

@zemaj any idea when this will be merged?

[stevehipwell]

@stefanprodan could someone take a look at this?

[stevehipwell]

Or @hiddeco?

[stevehipwell]

I could really do with knowing if this is going to be merged as if it's not we're going to have to come up with another solution such as a different Docker image. On it's own Kustomize is a really bad fit for a truly automated GitOps workflow and envsubst combined with the Flux extraEnvs solves this without moving over to something like ytt.

[stevehipwell]

Could someone take a look at this and let me know if this is going to be merged?

[kingdonb]

Greetings @stevehipwell I am reviewing all of the open PRs to see which ones can be included in a next Flux v1 release.

If you can rebase and amend with DCO, I can try to incorporate your change. Flux developers are focused on the next version of Flux, (Flux v2), but if this is still an outstanding issue for you, please let me know. I am not able decide what gets merged or which items get prioritized for a release. As Flux v2 approaches feature parity, we hope your needs are able to be met with the new version, but if there are outstanding issues that can still be solved easily in Flux v1, my aim is to help with that.

[stevehipwell]

@kingdonb I'll rebase this now.

[stevehipwell]

Replaced by PR #3407.

[kingdonb]

Thank you