Local domain resolution for parallel instances

[lasomethingsomething]

User story

As a developer running multiple Shopware instances in parallel, I want each instance reachable at a stable, dedicated local hostname (e.g. admin.instance1.shopware.local, admin.instance2.shopware.local) instead of differing ports, so I can work with several instances simultaneously without tracking which port maps to which shop.

Problem to solve

With multiple instances, port-based access (localhost:8000, :8001, …) is hard to keep straight and changes as ports shift to avoid collisions. Stable per-instance subdomains make each shop addressable and memorable, and the storefront/admin URLs stay consistent across restarts. This is the OrbStack domain-resolution approach applied to local Shopware dev.

Related to #939 (running multiple shops in parallel / changing ports), but a different mechanism: #939 resolves conflicts by changing ports; this proposes routing by hostname so port juggling is unnecessary.

Origin: suggestion from the SCD audience.

Acceptance criteria

For review and refinement:

• Each instance is reachable via a dedicated local hostname (e.g. <service>.<instance>.shopware.local).
• Hostnames resolve locally without manual /etc/hosts edits per instance.
• Admin and storefront are addressable per instance (e.g. admin.instance1.shopware.local).
• APP_URL / sales-channel domains are set consistently with the assigned hostname.
• The assigned hostname(s) are shown clearly after setup and discoverable later.
• Works alongside running multiple instances at once (no collision between instances).
• Documented: how hostnames are assigned and how to customize them.

Readiness checklist

• Acceptance criteria are clearly defined.
• Backward compatibility impact addressed.
• Documentation written.
• Tests added or adjusted accordingly.

[lasomethingsomething]

Next step: needs spike / technical solution about:

• what works across mac, linux, windows
• HTTP certs: might want HTTPS because of payments
• brainstorm other questions to set up the spike structure and strategy for figuring out what needs to be done and how