Add handler for brew build container task state change event
    
    In BrewSignRpmHandler, we record all the planned Docker image builds
    into database. They have the state PLANNED and they have the
    ArtifactBuild.dep_on set properly according to dependencies between
    these images.
    
    When the build container task state is changed in brew, we should update
    the build state in db, and if the task closed successfully, we should
    rebuild containers that depends on it. At this moment, we only log the
    builds which need to be rebuilt.