Fix legacy build with podman when there is output on stderr

[LewisGaul]

Fixes #401

When using 'ctr-mgr build --quiet':

• Docker only outputs image ID (good)
• Podman v3.4.2 outputs stderr from 'RUN' commands to stderr (hmm)
• Podman-remote outputs the stderr from podman to stdout (bad)

Solution for all these cases is to take the last line of the output.

Tested manually with podman-remote.

root@ubuntu:~/ctrbuild# cat Dockerfile
FROM alpine
RUN sh -c 'echo "hello" >&2'
root@ubuntu:~/ctrbuild# podman build . -q --no-cache
hello
93eb2c508f2ce21bd39bc86cfea804119a5302994cc8bd94168c46aa9d00ca3b
root@ubuntu:~/ctrbuild# docker build . -q --no-cache
sha256:4c79a365502b79abb668f06ae4ddde5bdfd893d7673c9b10f1987aee970130a0