keep the platform defined, in priority, via DOCKER_DEFAULT_PLATFORM o…

[glours]

…r the service.plaform one if no build platforms provided

What I did During a compose up when the DOCKER_DEFAULT_PLATFORM isn't set, the platform list is reset and the service.platform defined in the compose file isn't applied during the build step. Now the DOCKER_DEFAULT_PLATFORM is applied if defined otherwise the service.platform is used (if existing of course)

Be aware that both are defined and not targeting the same platform you will get the following message (current behaviour):

image with reference `image-name:tag` was found but does not match the specified platform: wanted `service.platform value`, actual: `DOCKER_DEFAULT_PLATFORM value`

Related issue fixes #9864

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