[Dockerfile] [KubeRay Dashboard] Fix Dockerfile warnings (ENV format, CMD JSON args)

[CheyuWu]

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KubeRay Component

dashboard

What happened + What you expected to happen

When building the image, the following warnings appear:

 3 warnings found (use docker --debug to expand):
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 48)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 70)
 - JSONArgsRecommended: JSON arguments recommended for CMD to prevent unintended behavior related to OS signals (line 74)

We should update the Dockerfile to follow best practices and remove these warnings.

Tasks:

• Update ENV statements to use key=value syntax
• Change CMD HOSTNAME="0.0.0.0" node server.js → CMD ["node", "server.js"]
• Verify image builds cleanly with no warnings

Related to #4161

Reproduction script

$ docker buildx build \
  -f dashboard/Dockerfile \
  -t quay.io/kuberay/dashboard:dev \
  --platform linux/amd64 \
  --load \
  ./dashboard

Anything else

No response

Are you willing to submit a PR?

• Yes I am willing to submit a PR!