docker pull: warn when pulled single-arch image does not match --platform
docker pull: warn when pulled single-arch image does not match --platform

This takes the same approach as was implemented on `docker build`, where a warning
is printed if `FROM --platform=...` is used (added in 399695305c67aae1010e6754f90f9078aa39e742)

Before:

    docker rmi armhf/busybox
    docker pull --platform=linux/s390x armhf/busybox

    Using default tag: latest
    latest: Pulling from armhf/busybox
    d34a655120f5: Pull complete
    Digest: sha256:8e51389cdda2158935f2b231cd158790c33ae13288c3106909324b061d24d6d1
    Status: Downloaded newer image for armhf/busybox:latest
    docker.io/armhf/busybox:latest

With this change:

    docker rmi armhf/busybox
    docker pull --platform=linux/s390x armhf/busybox

    Using default tag: latest
    latest: Pulling from armhf/busybox
    d34a655120f5: Pull complete
    Digest: sha256:8e51389cdda2158935f2b231cd158790c33ae13288c3106909324b061d24d6d1
    Status: Downloaded newer image for armhf/busybox:latest
    WARNING: image with reference armhf/busybox was found but does not match the specified platform: wanted linux/s390x, actual: linux/arm64
    docker.io/armhf/busybox:latest

And daemon logs print:

   WARN[2021-04-26T11:19:37.153572667Z] ignoring platform mismatch on single-arch image  error="image with reference armhf/busybox was found but does not match the specified platform: wanted linux/s390x, actual: linux/arm64" image=armhf/busybox

When pulling without specifying `--platform, no warning is currently printed (but we can add a warning in future);

    docker rmi armhf/busybox
    docker pull armhf/busybox

    Using default tag: latest
    latest: Pulling from armhf/busybox
    d34a655120f5: Pull complete
    Digest: sha256:8e51389cdda2158935f2b231cd158790c33ae13288c3106909324b061d24d6d1
    Status: Downloaded newer image for armhf/busybox:latest
    docker.io/armhf/busybox:latest

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

bearstech/moby,AkihiroSuda/docker,Mokolea/docker,bearstech/moby,jimjag/docker,nawawi/docker,nalind/docker,gnailuy/docker,flavio/docker,moby/moby,vdemeester/docker,djs55/docker,moul/docker,yosifkit/docker,thaJeztah/docker,trebonian/docker,moul/docker,thaJeztah/docker,paralin/docker,konstruktoid/docker-upstream,Mokolea/docker,konstruktoid/docker-upstream,bearstech/moby,vdemeester/docker,mahak/docker,cpuguy83/docker,Ramzec/docker,trebonian/docker,Ramzec/docker,thaJeztah/docker,mahak/docker,flavio/docker,djs55/docker,flavio/docker,gnailuy/docker,docker/docker,jthelin/docker,nalind/docker,jimjag/docker,paralin/docker,cpuguy83/docker,cpuguy83/docker,yosifkit/docker,Mokolea/docker,Ramzec/docker,trebonian/docker,flavio/docker,djs55/docker,cpuguy83/docker,moby/moby,nawawi/docker,gnailuy/docker,jthelin/docker,konstruktoid/docker-upstream,vdemeester/docker,Ramzec/docker,moul/docker,mahak/docker,konstruktoid/docker-upstream,moby/moby,AkihiroSuda/docker,Mokolea/docker,docker/docker,nalind/docker,trebonian/docker,jameszhan/docker,AkihiroSuda/docker,moby/moby,jameszhan/docker,moul/docker,yosifkit/docker,paralin/docker,gnailuy/docker,jameszhan/docker,nawawi/docker,bearstech/moby,thaJeztah/docker,jimjag/docker,mahak/docker,yosifkit/docker,jimjag/docker,nawawi/docker,nalind/docker,docker/docker,AkihiroSuda/docker,paralin/docker