Add support for new BUILD_PATH advanced configuration variable

[ajhyndman]

Summary

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

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

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.

---

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.

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[ajhyndman]

Bump. I'd love to get some feedback on this.

[dmaslov]

Merge this one pls

[JamesXNelson]

I really need this; I am the buildmaster of a project which assembles java, python, javascript, groovy and C#. All the other languages took me less than a day to make a perfect overlayfs + symlink build cache. javascript has been four days and counting, and I still don't have a solution (can't move build/ location, can't symlink things into place, can't use mount --bind.... it's a real mess). Please, please... conventions are not really conventions unless they are configurable. convention: a way in which something is usually done. If it can't be changed, it's not a convention, it's hard-coded, unconfigurable and frankly a massive PITA to try to support.

[Niesyto]

I'd love it

[stale]

This pull request has been automatically marked as stale because it has not had any recent activity. It will be closed in 5 days if no further activity occurs.

[ajhyndman]

Bump. I'd still love to get some feedback on this.

[AlliterativeAlice]

Want to give my vote of support to this too. Would be much more convenient than having to use pre/post-build scripts to shuffle folders around when I have other software in the project using the "build" folder.

[Ishaan28malik]

LGTM

[stale]

This pull request has been automatically marked as stale because it has not had any recent activity. It will be closed in 5 days if no further activity occurs.

[n3ps]

Bump. A lot of people have been waiting on this one.

[goldhand]

As mentioned in #3824 (comment) you are assuming that the ./build directory isn't used by another build tool (which is every package where I where I work). This would be REALLY nice to have :(

[drkstr101]

This is incredibly annoying. Now I need to filter out all the Gradle build artifacts that go in this folder, or override project conventions for this one little piece that used create-react-app. I suppose the proper solution is to eject, but who has time for all that?