refactor: move base image reference to Dockerfile

This will allow us to take advantage of automatic update software to auto-pull new versions when they are released to docker hub.

Dockerfile

@@ -0,0 +1 @@
+FROM redis:5.0.7

config

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
-export REDIS_IMAGE=${REDIS_IMAGE:="redis"}
-export REDIS_IMAGE_VERSION=${REDIS_IMAGE_VERSION:="5.0.7"}
+export REDIS_IMAGE=${REDIS_IMAGE:="$(awk -F '[ :]' '{print $2}' Dockerfile)"}
+export REDIS_IMAGE_VERSION=${REDIS_IMAGE_VERSION:="$(awk -F '[ :]' '{print $3}' Dockerfile)"}
 export REDIS_ROOT=${REDIS_ROOT:="$DOKKU_LIB_ROOT/services/redis"}
 export REDIS_HOST_ROOT=${REDIS_HOST_ROOT:=$REDIS_ROOT}