Core: Use fs-extra emptyDir so build works on docker volume

[russellwpatterson]

Issue:
When using a docker volume as an output directory, the build process fails with the following error:

[2020-12-17T18:59:46Z] info @storybook/react v6.1.11
--
  | [2020-12-17T18:59:46Z] info
  | [2020-12-17T18:59:46Z] info => Cleaning outputDir /js-tests/rp-storybook-upgrade-test
  | [2020-12-17T18:59:46Z] ERR! Error: EBUSY: resource busy or locked, rmdir '/js-tests/rp-storybook-upgrade-test'
  | [2020-12-17T18:59:46Z] ERR!  [Error: EBUSY: resource busy or locked, rmdir '/js-tests/rp-storybook-upgrade-test'] {
  | [2020-12-17T18:59:46Z] ERR!   stack: "Error: EBUSY: resource busy or locked, rmdir '/js-tests/rp-storybook-upgrade-test'",
  | [2020-12-17T18:59:46Z] ERR!   errno: -16,
  | [2020-12-17T18:59:46Z] ERR!   code: 'EBUSY',
  | [2020-12-17T18:59:46Z] ERR!   syscall: 'rmdir',
  | [2020-12-17T18:59:46Z] ERR!   path: '/js-tests/rp-storybook-upgrade-test'
  | [2020-12-17T18:59:46Z] ERR! }

What I did

I updated the directory removal to use fs-extra's .emptyDir instead of .remove which empties a folder instead of removing it.

I believe this should be considered for a 6.1.x patch since this bug breaks CI for 6.1.x if that outputDir is a docker volume.

Related issue: #13168

How to test

• Is this testable with Jest or Chromatic screenshots? no
• Does this need a new example in the kitchen sink apps? no
• Does this need an update to the documentation? no

If your answer is yes to any of these, please make sure to include it in your PR.

[shilman]

LGTM. thanks @russellwpatterson 💯

[petmat]

Hey @shilman me and my team have been waiting for this fix since issue #13168 and were hopeful that the next patch release would include it. Now 6.1.12 was just released and this seems not to be in it. Will we not have this as a patch release? Do we have to wait until 6.2?