Allows logs to use local time zone instead of UTC

Co-authored-by: Ludovic Fernandez <ldez@users.noreply.github.com>

docs/configuration/logs.md

@@ -292,3 +292,34 @@ This allows the logs to be rotated and processed by an external program, such as
 
 !!! note
     This does not work on Windows due to the lack of USR signals.
+
+## Time Zones
+
+The timestamp of each log line is in UTC time by default.
+
+If you want to use local timezone, you need to ensure the 3 following elements:
+
+1. Provide the timezone data into /usr/share/zoneinfo
+2. Set the environement variable TZ to the timezone to be used
+3. Specify the field StartLocal instead of StartUTC (works on default Common Log Format (CLF) as well as JSON)
+
+Example using docker-compose:
+
+```yml
+version: '3'
+
+services:
+  traefik:
+    image: containous/traefik:[latest stable version]
+    ports:
+      - "80:80"
+    environment:
+      - "TZ=US/Alaska"
+    command:
+      - --docker
+      - --accesslog
+      - --accesslog.fields.names="StartUTC=drop"
+    volumes:
+      - "/var/run/docker.sock:/var/run/docker.sock"
+      - "/usr/share/zoneinfo:/usr/share/zoneinfo:ro"
+```

middlewares/accesslog/logger_formatters.go

@@ -24,6 +24,8 @@ func (f *CommonLogFormatter) Format(entry *logrus.Entry) ([]byte, error) {
 	var timestamp = defaultValue
 	if v, ok := entry.Data[StartUTC]; ok {
 		timestamp = v.(time.Time).Format(commonLogTimeFormat)
+	} else if v, ok := entry.Data[StartLocal]; ok {
+		timestamp = v.(time.Time).Local().Format(commonLogTimeFormat)
 	}
 
 	var elapsedMillis int64

middlewares/accesslog/logger_formatters_test.go

@@ -2,6 +2,7 @@ package accesslog
 
 import (
 	"net/http"
+	"os"
 	"testing"
 	"time"
 
@@ -57,10 +58,34 @@ func TestCommonLogFormatter_Format(t *testing.T) {
 				BackendURL:             "http://10.0.0.2/toto",
 			},
 			expectedLog: `10.0.0.1 - Client [10/Nov/2009:23:00:00 +0000] "GET /foo http" 123 132 "referer" "agent" - "foo" "http://10.0.0.2/toto" 123000ms
+`,
+		},
+		{
+			name: "all data with local time",
+			data: map[string]interface{}{
+				StartLocal:             time.Date(2009, time.November, 10, 23, 0, 0, 0, time.UTC),
+				Duration:               123 * time.Second,
+				ClientHost:             "10.0.0.1",
+				ClientUsername:         "Client",
+				RequestMethod:          http.MethodGet,
+				RequestPath:            "/foo",
+				RequestProtocol:        "http",
+				OriginStatus:           123,
+				OriginContentSize:      132,
+				RequestRefererHeader:   "referer",
+				RequestUserAgentHeader: "agent",
+				RequestCount:           nil,
+				FrontendName:           "foo",
+				BackendURL:             "http://10.0.0.2/toto",
+			},
+			expectedLog: `10.0.0.1 - Client [10/Nov/2009:14:00:00 -0900] "GET /foo http" 123 132 "referer" "agent" - "foo" "http://10.0.0.2/toto" 123000ms
 `,
 		},
 	}
 
+	// Set timezone to Alaska to have a constant behavior
+	os.Setenv("TZ", "US/Alaska")
+
 	for _, test := range testCases {
 		test := test
 		t.Run(test.name, func(t *testing.T) {