We need to create userguide examples for Xds not involving Observability and currently only the csm observability example has a docker image.
Closes#37778
COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37778 from kannanjgithub:docker_xds_example_cpp bc91c8d5da04b1f6e9eef6e85c2f9a7d25fb716e
PiperOrigin-RevId: 678362755
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Closes#36359
PiperOrigin-RevId: 625183028
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