Add Dockerfile for container deployment

[gaujain-nv]

Is your feature request related to a problem?

Yes. Currently Slurm-web only provides deb/rpm packages. Deploying on Kubernetes or container platforms requires users to create their own Dockerfile.

Describe the solution you'd like

I'd like to contribute a Dockerfile that:

• Uses multi-stage build for optimized image size (~700MB)
• Supports both gateway and agent via SLURM_WEB_MODE env var
• Includes all dependencies (GTK/Pango, LDAP)
• Builds frontend from source

Describe alternatives you've considered

• Using package installation in a container (messy, large image)
• Third-party container images (none available)

Additional context

I've successfully deployed Slurm-web v6.0.0 on AWS EKS using this Dockerfile, monitoring a Slurm cluster via the Slinky operator.

I'm willing to submit a PR with:

• Dockerfile
• docker-compose.yml (optional)
• Documentation updates (optional)

I have signed the CLA and am ready to contribute.

Questions:

1. Is this contribution welcome?
2. Any preferences for base image or structure?
3. Would you like container images published to GHCR?

[rezib]

Hello @gaujain-nv, note this is a duplicate of #266. I will answer your questions in your PR #688.