tests/docker: run script use realpath instead of readlink

The alpine docker image only comes with busybox, which doesn't have the
'-e' option on its readlink, so change it to 'realpath' to avoid that
problem.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Lucas Mateus Castro (alqotel) <lucas.araujo@eldorado.org.br>
Message-Id: <20220922135516.33627-5-lucas.araujo@eldorado.org.br>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220929114231.583801-5-alex.bennee@linaro.org>

tests/docker/run

@@ -15,7 +15,7 @@ if test -n "$V"; then
     set -x
 fi
 
-BASE="$(dirname $(readlink -e $0))"
+BASE="$(dirname $(realpath $0))"
 
 # Prepare the environment
 export PATH=/usr/lib/ccache:/usr/lib64/ccache:$PATH