Add TARGET_PLATFORMS CI variable

variables:

  TARGET_PLATFORMS:

    description: Docker/OCI platform specifiers to build images for;

      comma separated

stages:

- check

- build

    --local dockerfile=.

    --opt target=$TARGET

    --output=type=image,name=$CI_REGISTRY_IMAGE/$TARGET:build-$CI_PIPELINE_IID,push=true

    ${TARGET_PLATFORMS:+--opt platform=$TARGET_PLATFORMS}

    ${REVISION:+--opt build-arg:REVISION=$REVISION}

    ${VERSION:+--opt build-arg:VERSION=$VERSION}