dockerfile: Switch base image for final build

dockerfile:  Switch base image for final build

Switching the base image for the final build to alpine as it still
provides a minimal interface, but has a mechanism for easily including
relevant CA certificates.  This is currently pinned to a tagged version,
though since none of the underlying mechanisms are used this should
balance both remaining stable, supported for a period of time, and
providing usable functionality.

Resolves #5970
Affects #5056