network: add --mac-address support to docker network connect

[ahmedharabi]

Add --mac-address support to docker network connect

Closes #51796

This change adds support for specifying a MAC address when connecting an existing container to a network using docker network connect, bringing feature parity with docker run --network.

The flag follows the same behavior as existing network-related options such as --ip and --ip6. The MAC address is parsed and validated client-side, then forwarded to the daemon via EndpointSettings.MacAddress.

This is particularly useful for macvlan networks, where explicit MAC address assignment is a common requirement.

Note: This change requires a companion update in moby/moby for the daemon to actually apply the MAC address during network connection.

---

What I did

• Added a --mac-address flag to docker network connect
• Implemented MAC address parsing and validation
• Passed the MAC address through EndpointSettings.MacAddress to the daemon

---

How I did it

• Introduced a new CLI flag consistent with existing network flags
• Validated input using net.ParseMAC
• Converted the value to network.HardwareAddr before sending it to the API
• Ensured invalid MAC addresses are rejected before making the API call

---

How to verify it

docker network create -d macvlan \
  --subnet=192.168.70.0/24 \
  --gateway=192.168.70.1 \
  -o parent=<interface> test-macvlan

docker run -d --name mysql3 mysql:8.0

docker network connect \
  --mac-address 02:42:ac:70:00:28 \
  test-macvlan mysql3

docker inspect mysql3 | jq '.[0].NetworkSettings.Networks."test-macvlan".MacAddress'