Separate build environment variables from runtime environment variables (or both)

It seems like any time an environment variable is updated, a re-deploy with full image re-build is required. This takes a long time. Re-deploys should be optimized as much as possible in case of urgent updates. The rebuild is probably required in case the new variables are needed during image building, but in most cases that is probably not needed. Suggest a way to identify variables that when changed, a new image build is required, vs variables that when changed, simply need the container's environment updated. Or, maybe this can be controlled via docker file layers by the customer, in which case this feature idea is unnecessary. But the goal would be to be able to update a "runtime" variable and redeploy quickly.