Part of #2993.

Add a new profile setting, stack.epr_proxy_to, that can be used to configure the Package Registry that the local instance uses to proxy its requests to for retrieving packages that are not available locally.

How to test

Using the flag

Start a local stack using the profile parameter, for example:

elastic-package stack up -d --provider compose -U stack.epr_proxy_to=https://epr-staging.elastic.co

Check the following:

• Command output includes this:

  - Proxy to https://epr-staging.elastic.co
  

• ~/.elastic-package/profiles/default/stack/Dockerfile.package-registry includes this line (assuming current profile is default):

  ENV EPR_PROXY_TO=https://epr-staging.elastic.co
  

• Go to https://localhost:4601, browse the integrations UI and check the logs of the Package Registry container, look for entries with "request.uri":"https://epr-staging.elastic.co....

Using the profile configuration file

Edit ~/.elastic-package/profiles/default/config.yml, or create it if it doesn't exist. Add this line:

stack.epr_proxy_to: "https://epr-staging.elastic.co"

Start a local stack using the profile parameter, for example:

elastic-package stack up -d --provider compose

Check the same things as in the previous case.