vtc: Skip u12 on Alpine Linux

I'm not sure why this test is suddenly failing on circleci, but it used
to pass when build jobs were running docker in docker, and it passes for
me in an alpine:latest container.

bin/varnishtest/tests/u00012.vtc

 varnishtest	"trivial run of varnishadm in pass mode"
 
+feature cmd "! grep -q ID=alpine /etc/os-release"
+
 server s1 {
 	rxreq
 	txresp