[k8s] Fix custom image parsing

[romilbhardwaj]

Our current custom image text parsing for k8s uses lstrip incorrectly. The arg to lstrip specifies a set of characters, not a string. As a result, any image starting with the letters d,o,c,k,e,r or a permutation of docker gets parsed incorrectly:

>>> "rayproject/ray".lstrip('docker:')
'ayproject/ray'

Tested with the sky launch this YAML:

resources:
  cloud: kubernetes
  image_id: docker:rayproject/ray:2.4.0-py310-cpu

[Michaelvll]

Good catch @romilbhardwaj! LGTM.