Merge velero restore helper into velero image

[ywk253100]

Currently, there are three binaries in Velero: velero, velero-helper and velero-restore-helper. velero and velero-helper are packaged in the same velero image, velero-restore-helper is in a separate velero-restore-helper image.

Velero image can be set by the --image option while there is no such option for velero-restore-helper, users need to set it via a configmap.

I'd like to suggest we merge the velero-restore-helper into the velero image so that users don't need to set the images with different ways.

Velero server can read the image info via the container field environment variable: https://kubernetes.io/docs/tasks/inject-data-application/environment-variable-expose-pod-information/#use-container-fields-as-values-for-environment-variables

What's more, this is helpful when Velero is consumed in the downstream project.
For example, when packaging the Velero as a Carvel package, Carvel tool (imgpkg) can move the package bundle into different registries and deploy the package without modifying the image reference in the yaml files.

Vote on this issue!

This is an invitation to the Velero community to vote on issues, you can see the project's top voted issues listed here.
Use the "reaction smiley face" up to the right of this comment to vote.

• 👍 for "I would like to see this bug fixed as soon as possible"
• 👎 for "There are more important bugs to focus on right now"

[kaovilai]

No performance impact if pull policy is IfNotExist if I understand correctly

[reasonerjt]

to fix #8229 I plan to add "restricted" security context to restore-helper by default, this may impact the image of velero if we want to combine these two images.
see #8491