Normalize network hostname lookup

[thromel]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Network attachments are allocated under the hostname supplied by the runtime, while DNS queries are canonical DNS names and may include a trailing dot. DNS hostnames are also case-insensitive. Because AttachmentAllocator keyed hostnames exactly as supplied, a container allocated as web could fail lookup when the embedded DNS path queried web..

This normalizes allocator keys by lowercasing hostnames and treating a single trailing dot as optional across allocate, lookup, and deallocate. The original hostname remains preserved in the returned attachment.

This is a narrow fix for the embedded DNS lookup path. First-class Compose-style bare service discovery from containers through the network gateway is a separate feature request: #1809. Related DNS discussion: #856.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

Checks run:

swift test --filter AttachmentAllocatorTest
swift test -c debug -Xswiftc -warnings-as-errors --filter AttachmentAllocatorTest

Additional manual validation:

• On released container 1.0.0, created a custom network and container; direct dig @127.0.0.1 -p 2053 <container-name> A returned no A record.
• With this patched debug build installed under a temporary /tmp install root, the same direct queries returned the allocated container IP for both <container-name> and <container-name>..