Add write by root permission to support Openshift

[chobostar]

This PR addresses next issues:

• https://github.com/OT-CONTAINER-KIT/redis/pull/25 https://github.com/OT-CONTAINER-KIT/redis/issues/24 "Fix: Make sure /etc/redis is writable by the root group"
• https://github.com/OT-CONTAINER-KIT/redis/pull/22 "fix the data directory permissions"

How I tested it

• build image with name redis7

  $ docker build . -t redis7

• run 2 containers - without and with 'runAsUser' option:

  $ docker run --rm --name test-uid -d redis7
  $ docker run --rm --name test-uid-1001 --user 1001 -d redis7```
  
  # make sure that config file and data directory is writeable
  $ docker exec test-uid bash -c "echo 1 > /etc/redis/redis.conf"
  $ echo $?
  0
  $ docker exec test-uid bash -c "echo 1 > /data/1"
  $ echo $?
  0
  
  # make sure that config file and data directory is writeable for '1001 user'
  $ docker exec test-uid-1001 bash -c "echo 1 > /data/1"
  $ echo $?
  0
  $ docker exec test-uid-1001 bash -c "echo 1 > /etc/redis/redis.conf"
  $ echo $?
  0

  no errors

• Let's check intended users:

  $ docker exec test-uid bash -c "echo $UID"
  1000
  
  $ docker exec test-uid-1001 bash -c "echo $UID"
  1000

  $ docker exec test-uid bash -c "whoami"
  redis
  
  $ docker exec test-uid-1001 bash -c "whoami"
  whoami: unknown uid 1001

  UIDs as expected

[chobostar]

Dear maintainers @shubham-cmyk @iamabhishek-dubey @sandy724

I respect your time and effort, but if you get a chance to look at this fix - Openshift users will be happy to use redis v7.

Thanks anyway!