Allow to force Yarn usage

[Embraser01]

This PR add a new environment variable that allows anyone to force the usage of yarn. It's a simple workaround of #196.

The current workaround for Yarn workspaces (here) only works with Yarn v1, Yarn v2 use yarn.lock to determine the root of the project and so, setting a empty yarn.lock in a specific folder will fail.

I choose the env var USE_YARN but it could be anything really 😄.

[ehmicky]

Hi @Embraser01,

Thanks a lot for working on this!
This makes sense to me.
@mheffner @vbrown608 What do you think?

Note: we would also need to:

• Update @netlify/build since it runs yarn detecting yarn.lock as well
• Add to the documentation website

[Embraser01]

Note: we would also need to:

• Update @netlify/build since it runs yarn detecting yarn.lock as well

Looking at the code for @netlify/build, it seems to be a little trickier to make it work with Yarn v2:

Some options have changed, --no-progress and --no-interactive don't exist anymore so this command will fail...

Furthermore, the --cache-folder option is not used anymore (there is actually a special case for Netlify to report it as a warning instead of an error here). The global cache folder used by Yarn 2 is ~/.yarn/berry/cache/ but can be moved with the YARN_GLOBAL_FOLDER env var (ref.).

[ehmicky]

Thanks for all that information! I have summarized it in a separate issue for @netlify/build only at netlify/build#1535. Could you please check it out and confirm I haven't missed anything?

Any help to improve Yarn support in the build-image is really appreciated!

[ehmicky]

This looks good to me!
@vbrown608 @mheffner What do you think?

[vbrown608]

Looks good. I suggest we prefix the environment variable like NETLIFY_USE_YARN to avoid the kind of collisions we saw with SITE_ID.

(It would be weird if someone has USE_YARN set and doesn't want to use yarn, but we might as well make it a practice going forward.)

[ehmicky]

Issue to add to documentation: https://github.com/netlify/docs/issues/632