Docker image for adelphi Python package

[absurdfarce]

Enable simple running of adelphi commands via Docker:

[mersault@linux adelphi]$ docker run --network=host adelphi:0.2.1 export-cql                                                                                                      
WARNING:cassandra.cluster:Downgrading core protocol version from 66 to 65 for 127.0.0.1:9042. To avoid this, it is best practice to explicitly set Cluster(protocol_version) to th
e version supported by your cluster. http://datastax.github.io/python-driver/api/cassandra/cluster.html#cassandra.cluster.Cluster.protocol_version                                
WARNING:cassandra.cluster:Downgrading core protocol version from 65 to 5 for 127.0.0.1:9042. To avoid this, it is best practice to explicitly set Cluster(protocol_version) to the
 version supported by your cluster. http://datastax.github.io/python-driver/api/cassandra/cluster.html#cassandra.cluster.Cluster.protocol_version                                 
INFO:cassandra.policies:Using datacenter 'datacenter1' for DCAwareRoundRobinPolicy (via host '127.0.0.1:9042'); if incorrect, please specify a local_dc to the constructor, or lim
it contact points to local cluster nodes                          
INFO:adelphi:Excluding system keyspaces system_auth,system_schema,system_distributed,system,system_traces,system_views,system_virtual_schema
INFO:adelphi:Processing the following keyspaces: foo,testkeyspace     
//{                                                                                                     
//    "host_count": 1,                                                                                             
//    "dc_count": 1,                                                                                                        
//    "cassandra_versions": "4.0-rc1",                                                        
//    "creation_timestamp": "2021-05-25T22:43:15.451119+00:00"                                                                                
//}  
...

This image should have utility both for the Argo workflow itself (which has to do "pip install" in a few places) and for end users who wish to try out Adelphi but don't want to muck around with Python packages.

[absurdfarce]

For Python3.x adelphi 0.2.1 will attempt to install PyGithub 1.55 (as of this writing) which in turn will try to install pynacl, a set of Python bindings for libsodium. pynacl requires native extensions but since gcc isn't present in the alpine image this will fail. pip will then fall back on the next PyGithub release, which is 1.54.1. We save ourselves that process by pre-emptively installing the proper PyGithub release before installing adelphi.