improve container detection using container specific files

proc/proc.go

@@ -150,6 +150,12 @@ func GetContainerRuntime(tgid, pid int) ContainerRuntime {
 		return runtime
 	}
 
+	// Check for container specific files
+	runtime = detectContainerFiles()
+	if runtime != RuntimeNotFound {
+		return runtime
+	}
+
 	return RuntimeNotFound
 }
 
@@ -167,6 +173,23 @@ func getContainerRuntime(input string) ContainerRuntime {
 	return RuntimeNotFound
 }
 
+// Related implementation: https://github.com/systemd/systemd/blob/6604fb0207ee10e8dc05d67f6fe45de0b193b5c4/src/basic/virt.c#L523-L549
+func detectContainerFiles() ContainerRuntime {
+	files := map[ContainerRuntime]string{}
+	// https://github.com/containers/podman/issues/3586#issuecomment-661918679
+	files[RuntimePodman] = "/run/.containerenv"
+	// https://github.com/moby/moby/issues/18355
+	files[RuntimeDocker] = "/.dockerenv"
+
+	for runtime, file := range files {
+		if fileExists(file) {
+			return runtime
+		}
+	}
+
+	return RuntimeNotFound
+}
+
 // GetContainerID returns the container ID for a process if it's running in a container.
 // If pid is less than one, it returns the container ID for "self".
 func GetContainerID(tgid, pid int) string {