Add docker-compose handler

This handler filters out the docker-compose prefix, and then passes on
the rest of the line to another handler.

docker_compose_handler.go

@@ -0,0 +1,23 @@
+package humanlog
+
+import (
+	"regexp"
+)
+
+// dcLogsPrefixRe parses out a prefix like 'web_1 | ' from docker-compose
+var dcLogsPrefixRe = regexp.MustCompile("^(?:\x1b\\[\\d+m)?([a-zA-Z0-9._-]+)\\s+\\|(?:\x1b\\[0m)? (.*)$")
+
+type handler interface {
+	TryHandle([]byte) bool
+	setField(key, val []byte)
+}
+
+func tryDockerComposePrefix(d []byte, nextHandler handler) bool {
+	if matches := dcLogsPrefixRe.FindSubmatch(d); matches != nil {
+		if nextHandler.TryHandle(matches[2]) {
+			nextHandler.setField([]byte(`service`), matches[1])
+			return true
+		}
+	}
+	return false
+}

json_handler.go

@@ -131,6 +131,12 @@ func (h *JSONHandler) UnmarshalJSON(data []byte) bool {
 
 	return true
 }
+func (h *JSONHandler) setField(key, val []byte) {
+	if h.Fields == nil {
+		h.Fields = make(map[string]string)
+	}
+	h.Fields[string(key)] = string(val)
+}
 
 // Prettify the output in a logrus like fashion.
 func (h *JSONHandler) Prettify(skipUnchanged bool) []byte {

scanner.go

@@ -42,6 +42,14 @@ func Scanner(src io.Reader, dst io.Writer, opts *HandlerOptions) error {
 			dst.Write(logfmtEntry.Prettify(opts.SkipUnchanged && lastLogfmt))
 			lastLogfmt = true
 
+		case tryDockerComposePrefix(lineData, &jsonEntry):
+			dst.Write(jsonEntry.Prettify(opts.SkipUnchanged && lastJSON))
+			lastJSON = true
+
+		case tryDockerComposePrefix(lineData, &logfmtEntry):
+			dst.Write(logfmtEntry.Prettify(opts.SkipUnchanged && lastLogfmt))
+			lastLogfmt = true
+
 		default:
 			lastLogfmt = false
 			lastJSON = false