Add volume mapping for Docker credentials

[style95]

Fix #4787

Additional information you deem important:

An operator can make invokers access to a private registry by adding this:

invoker_docker_volumes:
  - ~/.docker:/root/.docker

[codecov-io]

Codecov Report

Merging #4791 into master will decrease coverage by 6.64%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4791      +/-   ##
==========================================
- Coverage   85.18%   78.53%   -6.65%     
==========================================
  Files         197      197              
  Lines        8843     8843              
  Branches      610      610              
==========================================
- Hits         7533     6945     -588     
- Misses       1310     1898     +588

Impacted Files	Coverage Δ
...core/database/cosmosdb/RxObservableImplicits.scala	0% <0%> (-100%)	⬇️
...n/scala/org/apache/openwhisk/common/CausedBy.scala	0% <0%> (-100%)	⬇️
...enwhisk/connector/kafka/KamonMetricsReporter.scala	0% <0%> (-100%)	⬇️
...e/database/cosmosdb/cache/ChangeFeedConsumer.scala	0% <0%> (-100%)	⬇️
...ore/database/cosmosdb/cache/CacheInvalidator.scala	0% <0%> (-100%)	⬇️
...core/database/cosmosdb/CosmosDBArtifactStore.scala	0% <0%> (-96.23%)	⬇️
...sk/core/database/cosmosdb/CosmosDBViewMapper.scala	0% <0%> (-93.85%)	⬇️
...tabase/cosmosdb/cache/CacheInvalidatorConfig.scala	0% <0%> (-92.31%)	⬇️
...e/database/cosmosdb/cache/KafkaEventProducer.scala	0% <0%> (-77.78%)	⬇️
...whisk/core/database/cosmosdb/CosmosDBSupport.scala	0% <0%> (-74.08%)	⬇️
... and 10 more

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[style95]

It's ready to review.