Check if file exists before reading

src/Runner.Worker/ContainerOperationProvider.cs

@@ -62,11 +62,16 @@ namespace GitHub.Runner.Worker
                 throw new NotSupportedException("Container feature is not supported when runner is already running inside container.");
             }
 #else       
-            var initProcessCgroup = File.ReadLines("/proc/1/cgroup");
+            var path = "/proc/1/cgroup";
+            // OSX does not have this file, but you cannot run OSX as a base image for docker containers currently.
+            if (File.Exists(path))
+            {
+                var initProcessCgroup = File.ReadLines(path);
                 if (initProcessCgroup.Any(x => x.IndexOf(":/docker/", StringComparison.OrdinalIgnoreCase) >= 0))
                 {
                     throw new NotSupportedException("Container feature is not supported when runner is already running inside container.");
                 }
+            }
 #endif
 
 #if OS_WINDOWS