[dev.boringcrypto] crypto/internal/boring: update build instructions to use podman

[dev.boringcrypto] crypto/internal/boring: update build instructions to use podman

The 'docker' command line tool is no longer available on my Linux laptop
due to Docker's new licensing rules. 'sudo podman' seems to work fine,
so suggest that instead.

Change-Id: Ib80211404dadb567c8741720ece2a73a6ad4040a
Reviewed-on: https://go-review.googlesource.com/c/go/+/395874
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>