[gatsby-source-shopify] More clarity in docs around the BULK limitations and workaround

[njbmartin]

Hi team,

Our website uses gatsby-source-shopify and gatsby-source-contentful (among others), and we use Gatsby Cloud with CMS Preview enabled. Due to the nature of CMS preview, it's very likely that Production + CMS Preview will be triggered at the same time if the Gatsby Cloud plan has build concurrency.

A workaround was identified where each environment / preview / developer, should have their own Custom App. This workaround should be added to the documentation to avoid people running into this issue.

This doesn't just apply to Gatsby Cloud as it's any build environment that has concurrency (eg. testing), but Gatsby Cloud + CMS Preview is a prime example of where this workaround needs to be applied.

[njbmartin]

Think also the timeout / retry logic needs reworking... This had been holding up 7 queued builds! Been stuck for nearly 2 hours.