"man podman-build" suggests that the context argument is optional:
podman build [options] [context]
podman image build [options] [context]
If no context directory is specified, then Podman will assume
the current working directory as the build context, which
should contain the Containerfile.
but if i have a directory with nothing but a Containerfile, i get:
$ podman build
Error: no context directory specified, and no containerfile specified
OTOH, specifying context of current directory:
$ podman build .
STEP 1: FROM alpine:latest
... etc etc ...