Add `REDIS_HOST` and `POSTGRES_HOST` option from env

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Please provide the following information so we can assess the issue you're having

Description

Redis and postgres hosts are hard-coded. Add option to override them.

Describe the results you received:

Testing application on EKS cluster with ElastiCache for Redis and RDS db for postgres.

Describe the results you expected:

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

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Output of docker info:

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Additional environment details (AWS, Docker for Mac, Docker for Windows, VirtualBox, physical, etc.):

dockersamples / example-voting-app