Update OpenShift Dockerfile
    
    This is part of the work to create container images suitable for integration testing:
    
     * Adjust Dockerfile to be in sync with the ones from WaiverDB and Greenwave.
     * Add documentation on how to build and use the image, and the OpenShift template.
    
    Build OpenShift container image on Fedora 28
    --------------------------------------------
    
    This will sync the container image to be used in OpenShift
    deployements with WaiverDB and Greenwave.
    
    mod_wsgi-express is provided by mod_wsgi in the Fedora repos
    now, so remove installing it from PyPi and use dnf instead.
    
    Allow resultsdb_rpm build arg to be a URL
    --------------------------------------------
    
    Remove USER directives
    --------------------------------------------
    
    Follow OpenShift guidelines [0], and adjust the group
    ownership of the start-up script, instead of manipulating the
    user ID.
    
    [0]
    <https://docs.openshift.org/latest/creating_images/guidelines.html#openshift-specific-guidelines>
    
    Document usage of the OpenShift container image
    --------------------------------------------
    
    Add description and usage labels, and define expectations for
    volume mounts.
    
    Add pre-deployment hook
    --------------------------------------------
    
    Using pre-deployment hooks to initialise the database is
    safer and the right way to go.
    See: https://pagure.io/waiverdb/issue/121
    
    In order handle timeouts due to the database service not being
    available as the hook is executed, `resultsdb init_db`
    is being re-triggered for up to 10 minutes, until succeeds.
    
    Modify container entrypoint
    --------------------------------------------
    
    As the database initialisation is triggered as a
    pre-deployement hook, there is no need for the run_app
    script anymore.
    
    This also tries to follow recommendations for
    ENTRYPOINT and CMD usage, and use CMD as a way to define
    default arguments for the ENTRYPOINT.
    
    Signed-off-by: Hunor Csomortáni <csomh@redhat.com>
    
    Merges #111