Allow building a Dockerfile (instead of a directory) and using docker buildx build explicitly

[tianon]

Add support for --build=Dockerfile

The --build flag currently supports a directory to build - this adds support for specifying a file instead (thus with no directory context).

This also suppots Bash's "process substitution" such that something like the following is doable (to inline a Dockerfile to build):

hocker_run --build <(printf 'FROM ...\nRUN ...\n') ...

Allow --buildx as an alternate to --build

This will build with docker buildx build explicitly.

[tianon]

Most of this (all the Dockerfile stuff) is code I've been using happily on one of my personal systems for a really long time now.

However, the buildx stuff is brand new and I've realized it's actually wrong thanks to buildx not necessarily using images from docker pull (so us doing the pulling isn't necessarily right). 🙃