fix(docker): restore NEXT_PUBLIC_APP_URL build arg with dummy fallbac… #17223

Job	Run time
Detect Version	3s
Test and Build / Test and Build	5m 42s
Check Docs Changes	0s
Build AMD64 (./docker/app.Dockerfile, ghcr.io/simstudioai/simstudio, ECR_APP)	3m 34s
Build AMD64 (./docker/db.Dockerfile, ghcr.io/simstudioai/migrations, ECR_MIGRATIONS)	26s
Build AMD64 (./docker/realtime.Dockerfile, ghcr.io/simstudioai/realtime, ECR_REALTIME)	28s
Build ARM64 (GHCR Only)	0s
Build Dev ECR	0s
Migrate Dev DB	0s
Process Docs	0s
Create GHCR Manifests	0s
Create GitHub Release	0s
	10m 13s

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