Fix Dockerfile to handle non-unique GID for macOS compatibility

[Prajeet-Shrestha]

Fixes #656

Problem

On macOS, most user belongs to a group which has GID 20. Sandcastle passes this GID to docker build, but inside the node:22-bookworm image, GID 20 is already used by another group, because of that it was causing the issue.

Fix

Added --non-unique to both groupmod and usermod in Dockerfile for all the agents.

Before:
RUN groupmod -g $AGENT_GID node && usermod -u $AGENT_UID -g $AGENT_GID -d /home/agent -m -l agent node

After:
RUN groupmod -g $AGENT_GID --non-unique node && usermod -u $AGENT_UID -g $AGENT_GID -d /home/agent -m -l agent --non-unique node

[vercel]

@Prajeet-Shrestha is attempting to deploy a commit to the Matt Pocock's projects Team on Vercel.

A member of the Team first needs to authorize it.

[hlynurstef]

I've already put up a PR with the same fix: #613