Empty build output directory instead of remove

[remingtonc]

Summary

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

This PR simply changes fs.remove during build/build.js to fs.emptyDir. This arose due to a semi-funky build system in which we are building the documentation via Docker container into a Docker volume which is also attached to an NGINX container which has the .vuepress/dist folder mounted as a Volume and served via NGINX. fs.remove attempts to unlink the dist folder, causing a build failure due to the NGINX mount. emptyDir simply empties the directory and does not delete it, and I couldn't obviously see a dependency on totally removing the .vuepress/dist folder.

Unfortunately there was a significant amount of test failures, but none that looked suspiciously due to this change:

Test Suites: 27 failed, 3 passed, 30 total
Tests:       4 failed, 8 passed, 12 total
Snapshots:   0 total
Time:        6.872s
Ran all test suites.

Many due to:

 FAIL  packages/@vuepress/shared-utils/__tests__/fileToPath.spec.js
  ● Test suite failed to run

    TypeError: programPath.hub.addHelper is not a function

      at wrapInterop (node_modules/@babel/plugin-transform-modules-commonjs/node_modules/@babel/helper-module-transforms/lib/index.js:165:45)
      at PluginPass.exit (node_modules/@babel/plugin-transform-modules-commonjs/lib/index.js:174:70)
      at newFn (node_modules/@babel/traverse/lib/visitors.js:193:21)
      at NodePath._call (node_modules/@babel/traverse/lib/path/context.js:53:20)
      at NodePath.call (node_modules/@babel/traverse/lib/path/context.js:40:17)
      at NodePath.visit (node_modules/@babel/traverse/lib/path/context.js:97:8)
      at TraversalContext.visitQueue (node_modules/@babel/traverse/lib/context.js:118:16)
      at TraversalContext.visitSingle (node_modules/@babel/traverse/lib/context.js:90:19)
      at TraversalContext.visit (node_modules/@babel/traverse/lib/context.js:146:19)
      at Function.traverse.node (node_modules/@babel/traverse/lib/index.js:94:17)

Built and run in Docker:

FROM node:alpine
COPY . /data/
WORKDIR /data/
ENTRYPOINT [ "sh" ]
CMD [ "test.sh" ]

#!/usr/bin/env sh
yarn
yarn test
yarn dev

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[ulivz]

LGTM