Use correct Make target in PR preview workflow

[marc1404]

How to categorize this PR?

/kind bug cleanup

What this PR does / why we need it:

Main motivation:

• Using the correct Make target in the PR preview workflow.

Other changes:

• Removed the scripts section in the package.json. This is a bit opinionated, but it doesn't add value if it's just redirecting to Make targets anyway.
• Fixed the Docker build and documented the correct Make target for running the Docker preview.
• Pinned the dependencies in the package.json. This is a Node.js best practice.

Which issue(s) this PR fixes:

n.a.

Special notes for your reviewer:

/cc @klocke-io

[gardener-prow]

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[BoHristova]

Thank you! Just today, I was struggling to understand why make docker-preview doesn't work for me, and @klocke-io suggested some of the changes included in this PR. I was about to open a PR myself when I saw that you had already fixed the issue, updated the README, and even went the extra mile. Great work!

/lgtm

[gardener-prow]

LGTM label has been added.

Details

Git tree hash: e66ac717b48b14b085d46aba4b5859d1328b8c0f

[marc1404]

Merging to fix the documentation build & deploy.