[chore] frontend: update Dockerfile to reduce final image size by 15%

[macno]

Changes

Currently, all the dev dependencies are included in the final image.

Reworking the first two stages, we can reduce the final image by ~15%

╰─➤  docker images otel-demo-frontend
REPOSITORY           TAG       IMAGE ID       CREATED          SIZE
otel-demo-frontend   fix       0a53d5f0fdcc   14 minutes ago   707MB
otel-demo-frontend   orig      a863b800e2ab   19 minutes ago   823MB

Merge Requirements

For new features contributions, please make sure you have completed the following
essential items:

• CHANGELOG.md updated to document new feature additions
• Appropriate documentation updates in the docs
• Appropriate Helm chart updates in the helm-charts

Maintainers will not merge until the above have been completed. If you're unsure
which docs need to be changed ping the
@open-telemetry/demo-approvers.