Similar to how [distribution.newRepository] in the legacy distribution code
passes the (custom) http-headers. User-Agent is always set, and can't be
overridden, so we apply it after setting the custom headers.

[distribution.newRepository]: https://github.com/moby/moby/blob/9ce272f804a9383c9aa593527d318c946bfb7b38/daemon/internal/distribution/registry.go#L74-L97

Before this patch:

    docker run --rm -d --name debugger -p 127.0.0.1:5001:8080 mendhak/http-https-echo
    DOCKER_CUSTOM_HEADERS=X-Meta-Hello=thaJeztah docker pull localhost:5001/myimage:latest
    docker logs debugger
    ...
    "headers": {
        "host": "localhost:5001",
        "user-agent": "docker/dev go/go1.24.7 git-commit/8e89fe7e8cbb3048f640846590175cbae4719b25 kernel/6.10.14-linuxkit os/linux arch/arm64 containerd-client/2.1.4+unknown storage-driver/overlayfs UpstreamClient(Docker-Client/28.3.2 \\(linux\\))",
        "accept": "application/json, */*",
        "accept-encoding": "zstd;q=1.0, gzip;q=0.8, deflate;q=0.5",
        "baggage": "trigger=api"
    },

With this patch:

    docker run --rm -d --name debugger -p 127.0.0.1:5001:8080 mendhak/http-https-echo
    DOCKER_CUSTOM_HEADERS=X-Meta-Hello=thaJeztah docker pull localhost:5001/myimage:latest
    docker logs debugger
    ...
    "headers": {
        "host": "localhost:5001",
        "user-agent": "docker/dev go/go1.24.7 git-commit/8e89fe7e8cbb3048f640846590175cbae4719b25 kernel/6.10.14-linuxkit os/linux arch/arm64 containerd-client/2.1.4+unknown storage-driver/overlayfs UpstreamClient(Docker-Client/28.3.2 \\(linux\\))",
        "accept": "application/json, */*",
        "accept-encoding": "zstd;q=1.0, gzip;q=0.8, deflate;q=0.5",
        "baggage": "trigger=api",
        "x-meta-hello": "thaJeztah"
    },

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>