[studio] CI workflow fails at startup due to unapproved Docker actions

## Problem

The CI workflow (.github/workflows/ci.yml) fails at startup with "No jobs were run" on every push and pull request. This means none of the four jobs - backend build, backend tests, frontend build, or frontend Docker build - actually execute.

## Root cause

The rontend-docker-build job references two third-party GitHub Actions that are not on the Apache organization's action allowlist:

`yaml
- name: Set up Docker Buildx
uses: docker/setup-buildx-action@v3 # line 88

- name: Build frontend Docker image (cached)
uses: docker/build-push-action@v6 # line 91
`

When GitHub Actions encounters an action that is not approved by the org admin, it blocks the entire workflow run - not just the offending job. The result is that all CI checks report as "failed" with no steps executed.

## Impact

- No backend or frontend builds are validated on any PR or push to
ocketmq-studio.
- No backend tests run in CI.
- Contributors cannot see whether their changes pass or fail CI - the status is always a startup failure.
- The rontend-docker-build job's purpose is to verify that web/Dockerfile builds successfully. This check is currently never performed.

## Proposed fix

Replace the third-party Docker actions with a plain docker build command. This removes the buildx layer (and its GHA cache) but allows the workflow to run normally. The Dockerfile is a simple two-stage build that does not require buildx features.

`yaml
- name: Build frontend Docker image
run: docker build -t rocketmq-studio-web-ci:latest ./web
`

This change unblocks all four CI jobs and restores visibility into build/test status for every PR.

Start with .github/workflows/ci.yml, especially the frontend-docker-build job and the Docker action references around lines 88 and 91. Review web/Dockerfile and run the workflow on a push or pull request. Done means all four jobs start normally and the frontend Docker image builds successfully.

Written by the indexing model from the issue text.