preparations for soft deprecation of legacy notification args (#1377)

Co-authored-by: Simon Aronsson <simme@arcticbit.se>

pkg/container/cgroup_id.go

@@ -0,0 +1,29 @@
+package container
+
+import (
+	"fmt"
+	"os"
+	"regexp"
+
+	"github.com/containrrr/watchtower/pkg/types"
+)
+
+var dockerContainerPattern = regexp.MustCompile(`[0-9]+:.*:/docker/([a-f|0-9]{64})`)
+
+// GetRunningContainerID tries to resolve the current container ID from the current process cgroup information
+func GetRunningContainerID() (cid types.ContainerID, err error) {
+	file, err := os.ReadFile(fmt.Sprintf("/proc/%d/cgroup", os.Getpid()))
+	if err != nil {
+		return
+	}
+
+	return getRunningContainerIDFromString(string(file)), nil
+}
+
+func getRunningContainerIDFromString(s string) types.ContainerID {
+	matches := dockerContainerPattern.FindStringSubmatch(s)
+	if len(matches) < 2 {
+		return ""
+	}
+	return types.ContainerID(matches[1])
+}