Universal Docker app for BOINC central

[davidpanderson]

I propose that we add a 'universal Docker app' to BOINC central.

The web interface for submitting a batch of jobs:

    select (from file sandbox)
        main program (executable, possibly a script)
        Dockerfile
            RUN must run main prog
        additional files
        a .tgz file with a directory of per-job directories
            job1/
                [cmdline1]
                infile1
                infile2
            job2/
                [cmdline2]
                infile1
                infile2
    additional cmdline (passed to all jobs)
    the name of the output file of each job (default 'out')

The output file of a job can be a single file,
or a .tgz or .zip of multiple files.