Building VRT docker image fails when `ROOT_DIR` contains a space

[derweili]

Bug Description

When using a local environment like Local, WordPress lives in a Local Sites folder that has a space in it's name.

This space in the path name causes bin/backstop to fail building the docker image with following error:

ERROR: "docker buildx build" requires exactly 1 argument.
See 'docker buildx build --help'.

Steps to reproduce

1. Clone the Site Kit repo in a folder with a space in its path and install the dependencies.
2. Verify the googlesitekit/vrt:* image does not exist on your machine.
3. Run test:visualtest
4. See the above error

Additional Context

• OS: macOS 14.1

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Acceptance criteria

• The bin/backstop script (also callable via the backstopjs npm script) should run consistently regardless of the presence of whitespace in its absolute path to the file

Implementation Brief

• Update bin/backstop to address all ShellCheck errors and warnings, particularly https://www.shellcheck.net/wiki/SC2086

Test Coverage

• No changes

QA Brief

• Check out the Site Kit repo into a folder that contains a space " " character
• Run docker rmi googlesitekit/vrt:6.1.1 to remove the VRT image from your system's Docker images
• Run npm install, then npm run backstopjs from the command line to build the docker image
• It should build without issue

Changelog entry

• N/A

[tofumatt]

IB ✅

[10upsimon]

ENG:QA Update - Pass ✅

• ✅ Checked out into folder with space, /Users/[REDACTED]/Downloads/sk test
• ✅ Removed existing Backstop image
• ✅ Ran npm install
• ✅ Ran npm run backstopjs
• ✅ Image built with no problems and Backstop CLI welcome message was visible