Commit 21e50b8 added a label on the buildkit
worker to advertise the host-gateway-ip. This option can be either set by the
user in the daemon config, or otherwise defaults to the gateway-ip.

If no value is set by the user, discovery of the gateway-ip happens when
initializing the network-controller (`NewDaemon`, `daemon.restore()`).

However d222bf0 changed how we handle the
daemon config. As a result, the `cli.Config` used when initializing the
builder only holds configuration information form the daemon config
(user-specified or defaults), but is not updated with information set
by `NewDaemon`.

This patch adds an accessor on the daemon to get the current daemon config.
An alternative could be to return the config by `NewDaemon` (which should
likely be a _copy_ of the config).

Before this patch:

    docker buildx inspect default
    Name:   default
    Driver: docker

    Nodes:
    Name:      default
    Endpoint:  default
    Status:    running
    Buildkit:  v0.12.4+3b6880d2a00f
    Platforms: linux/arm64, linux/amd64, linux/amd64/v2, linux/riscv64, linux/ppc64le, linux/s390x, linux/386, linux/mips64le, linux/mips64, linux/arm/v7, linux/arm/v6
    Labels:
     org.mobyproject.buildkit.worker.moby.host-gateway-ip: <nil>

After this patch:

    docker buildx inspect default
    Name:   default
    Driver: docker

    Nodes:
    Name:      default
    Endpoint:  default
    Status:    running
    Buildkit:  v0.12.4+3b6880d2a00f
    Platforms: linux/arm64, linux/amd64, linux/amd64/v2, linux/riscv64, linux/ppc64le, linux/s390x, linux/386, linux/mips64le, linux/mips64, linux/arm/v7, linux/arm/v6
    Labels:
     org.mobyproject.buildkit.worker.moby.host-gateway-ip: 172.18.0.1

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 00c9785)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>