[Eui+] Fix docusaurus containerId for local environment

[mgadewoll]

Summary

This PR ensures that Docusaurus can run locally as expected when no GTM containerId is available. This is a small update to the changes done in this PR.

This affects both dev environment and production build BUT locally only.
There is no issue with the production build in CI as the containerId is provided in the pipeline from the vault.

QA

• checkout the PR and verify the website can be run locally without error
  • gh pr checkout 7938
  • go to /packages/website
  • install all local dependencies yarn workspaces foreach -Rpti --from @elastic/eui-website run build
  • start the local dev environment yarn start
  • build production locally and run it: yarn build && yarn serve

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: f53e501

[tkajtoch]

LGTM