[Docker] Fix incomplete Dockerfile and update README with local Docker instructions

### Problem
The current Dockerfile does not install dependencies (yarn install) or build the app (yarn build), making the container non-functional when built locally.

### Impact
- Docker images built locally fail to run (missing node_modules).
- Contributors cannot run the app via Docker without manual workarounds.
- GitHub Actions builds the app separately, but the image doesn’t include these steps.
- Violates the principle of portable containers.

### Proposed Solution
- Update Dockerfile
- Install dependencies explicitly (yarn install).
- Build the app inside the container (yarn build).
- Remove redundant build steps from GitHub Actions.
- Update README.md

Hi @aryanmehrotra @aditya-tiwari-zs @satyamt05 👋.
I’d like to work on this. Please assign this issue to me.