Add support for new BUILD_PATH advanced configuration variable

Created by: ajhyndman

Summary

Adds support for a new environment variable, BUILD_PATH, which overrides the create-react-app output directory.

Fixes #378 Fixes #778 (closed) Fixes #1354 (closed) Fixes #3824 (closed) Fixes #3973 (closed) Fixes #6211 (closed) Relates to #7802 Fixes #8600 (closed)

Rationale

First, I know, this is a hot button issue. I have been following create-react-app for years now, and I understand and support the objective of providing a configuration-free setup experience for new React projects. However, I hope you'll allow me a few paragraphs to make an argument for this change.

I think there are several qualities about build path configuration that make it a good candidate as an addition to create-react-app's set of "advanced configuration" environment variables:

• It's trivial to support. Including documentation, this whole diff is less than 10 lines of code.
• CRA is very hands-off about what you do with build output, so this change is unlikely to add a maintenance burden.
• It still generates issues and support requests.
• There are real compatibility issues that existing recommendations can't resolve.
  • The mv command is not cross-platform compatible
  • The 'build' path can conflict with other tooling
  • Symlink-based build systems are hard to support
• A large number of users describe or recommend ejecting solely for the purpose of changing output path.

In summary, I think that a customisable output path is: cheap to support, won't add to maintenance burden, reduces ongoing support work, and allows significantly more projects to avoid ejecting and benefit from future CRA updates. Together, I think these qualities make a new BUILD_PATH environment variable a good candidate for support.

Verification

To verify this change works, I followed the guide in CONTRIBUTING.md and bootstrapped a project locally.

$ cd create-react-app 
$ yarn
$ yarn create-react-app my-app

In that project, I verified the following configuration cases:

1. With no configuration, files are still output to /build:

2. With the unprefixed folder name 'dist', files are output to /dist:

3. With a parent-relative path specified, files are output to the correct location:

In the third case, I also verified running yarn build multiple times correctly cleans up files before outputting a new build.

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Thank you to the create-react-app team being such exemplary members of the React and open source communities, and I appreciate you considering my pull request.