Use basic python base image in sample pipeline

[gmfrasca]

Discovered/mentioned in an unrelated pr (#611), the sample pipelines are using a 'custom' base image which may be misleading for users, suggesting that DSP only works with these images and not basic upstream python ones. Should we update these to use more well-known images?

comment text:

we should just use python base here

the docker rate limiting is a pain, but I also don't want to mislead people into thinking they need to use some special base image ... so feel free to point to a quay python if such a thing exists

Originally posted by @gregsheremeta in #611 (comment)