Add --seccomp flag to ctr

This enables testing of containers with the default seccomp profile

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

contrib/seccomp/seccomp_default_unsupported.go

@@ -0,0 +1,26 @@
+// +build !linux
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package seccomp
+
+import specs "github.com/opencontainers/runtime-spec/specs-go"
+
+// DefaultProfile defines the whitelist for the default seccomp profile.
+func DefaultProfile(sp *specs.Spec) *specs.LinuxSeccomp {
+	return &specs.LinuxSeccomp{}
+}