Fill application template environment from .env file

[musilto9]

Is your feature request related to a problem? Please describe

I have my own application templates loaded from a git repo.
When I want to deploy a stack from one of these templates I have to fill each environment variable manually, even though there is a method to set environment variables from a file when creating new stack.

Describe the solution you'd like

Add button "Load variables from .env file" to the configuration widget when deploying application template.
Instead of adding all variables in uploaded file it would only set predefined variables from the template.
This would improve the feature parity between deploying stack from custom templates and from application templates.

Describe alternatives you've considered

One I can find working right now is uploading all my application templates as custom templates from git repo manually (and keep updating them) and then creating a new stack from that template, but that seems quite cumbersome.
Alternatively adding the option to deploy from application template on the "Create stack" screen and then loading variables from env file would also work.

Additional context

No response