feat: support setting `GH_APP` in docker

[imnotjames]

Description

adds support for CODECOV_GITHUB_APP docker environment variable

fixes #2711

Code Example

docker -e CODECOV_GITHUB_APP=codecov-example some-image-reference

Notable Changes

When setting the CODECOV_GITHUB_APP, the github app linked out to when adding organizations is correct

Screenshots

N/A

Link to Sample Entry

N/A

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. In 2022 this entity acquired Codecov and as result Sentry is going to need some rights from me in order to utilize my contributions in this PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

[imnotjames]

It sets the name of the app when you're connecting a new organization. Without this set, you'll be directed to connecting codecov which is not the GitHub app for a self hosted instance