document how to consume secrets via docker

[edublancas]

from @idomic:

https://docs.cloud.ploomber.io/en/latest/user-guide/dockerfile.html
The guide above only mentions not to store secrets in the img, but it doesn't say how I consume a secret via the dockerfile.

[edublancas]

@idomic I'm a bit confused about this issue.

secrets are not available in the docker build step, they're only available at runtime, right?

so I'm unsure what the docs should say

[idomic]

Right, it should specify how users can consume secrets in their docker app.

Something along the lines of you add a secret like you'd normally do in the frameworks, and then in your docker file do X Y Z, and you should have that as an env variable available to your app.