Set imagePullPolicy in manager.yaml

[evalaiyc98]

Why are these changes needed?

Noticed that using make deploy command to deploy kuberay operator, it will use the default image tag :latest to build the operator.
In Kubernetes documentation, it said that

If don't specify a registry hostname, Kubernetes assumes that you mean the Docker public registry.

If omit the imagePullPolicy field, and the tag for the container image is :latest, imagePullPolicy is automatically set to Always.

To avoid the failure of pulling an image from a remote repository that does not exist, there should specify the imagePullPolicy.

Without this PR, there would be an error when deploying the kuberay operator.

With this PR, the process of deploying the kuberay operator is quick fast because can use the image which is already on the machine.

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Checks

• I've made sure the tests are passing.
• Testing Strategy
  • Unit tests
  • Manual tests
  • This PR is not tested :(