[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 <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: rsc

src/crypto/internal/boring/Dockerfile

@@ -6,10 +6,12 @@
 # Security Policy. To use it, build the image, run it, and then extract
 # /boring/godriver/goboringcrypto_linux_amd64.syso.
 #
-#   $ docker build -t goboring:140sp3678 .
-#   $ docker run -it --name goboring-140sp3678 goboring:140sp3678
-#   $ docker cp goboring-140sp3678:/boring/godriver/goboringcrypto_linux_amd64.syso .
+#   $ podman build -t goboring:140sp3678 .
+#   $ podman run -it --name goboring-140sp3678 goboring:140sp3678
+#   $ podman cp goboring-140sp3678:/boring/godriver/goboringcrypto_linux_amd64.syso .
 #   $ sha256sum goboringcrypto_linux_amd64.syso # compare to docker output
+#
+# The podman commands may need to run under sudo to work around a subuid/subgid bug.
 
 FROM ubuntu:focal