Add support for redis-stack containers

[crcarlo]

Using the current version of RedisContainer with redis/redis-stack-server or redis/redis-stack images, the container lacks Redis modules because start method overrides the entrypoint script of redis-stack.

This problem has been described in this thread #827 (comment)

This PR:

• Uses default redis-stack entrypoint script on start
• Adds a test for redis-stack with JSON module
• Updates the documentation to show the test as an example

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