Add envsubst dependency to commodore image

[thobens]

This PR adds the envsubst dependency from https://github.com/a8m/envsubst to the docker image, so it can be used in the CI pipelines of commodore components. The following PRs are dependent on this change:

• Basic implementation of the capi-core component component-capi-core#1
• Initial implementation using kustomize and envsubst vshn/component-capi-provider-cloudscale#1
• Initial implementation using kustomize and envsubst vshn/component-capi-provider-talos#1

NOTE: Check if it would be better to integrate this dependency in the commodore tools command and whether this would be out of scope for ALDE-262

Checklist

• Keep pull requests small so they can be easily reviewed.
• Update the documentation.
• Update tests.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, enhancement, documentation, change, breaking, dependency, internal
  as they show up in the changelog
• Link this PR to related issues.

[simu]

To get renovate support here, we need a new entry in renovate.json, cf.

commodore/renovate.json

Lines 51 to 56 in cd33baa

	{
	"fileMatch": ["^Dockerfile$"],
	"matchStrings": ["ARG HELM_VERSION=(?<currentValue>.*?)\\n"],
	"datasourceTemplate": "github-releases",
	"depNameTemplate": "helm/helm"
	}