get docker pull secrets from environment variables, if it's not existed, return [] image pull secret format in ENV: str1,str2,str3.

GetImageRepoFromEnv parse the image from environment variables, if it's not existed, return the default value.

GetImageTagFromEnv parse the image tag from environment variables, if it's not existed, return the default value.

GetWorkerImage get the image of alluxio worker from alluxioruntime, env or default TODO: Get image by calling runtime controller interface instead of reading runtime object.

ParseDockerImage extracts repo and tag from image.

ParseInitImage parses the init image and image tag.