add log rotation to docker driver log defaults

drivers/docker/driver.go

@@ -740,6 +740,15 @@ func (d *Driver) createContainerConfig(task *drivers.TaskConfig, driverConfig *T
 		Config: driverConfig.Logging.Config,
 	}
 
+	if hostConfig.LogConfig.Type == "" && hostConfig.LogConfig.Config == nil {
+		logger.Trace("no docker log driver provided, defaulting to json-file")
+		hostConfig.LogConfig.Type = "json-file"
+		hostConfig.LogConfig.Config = map[string]string{
+			"max-file": "2",
+			"max-size": "2m",
+		}
+	}
+
 	logger.Debug("configured resources", "memory", hostConfig.Memory,
 		"cpu_shares", hostConfig.CPUShares, "cpu_quota", hostConfig.CPUQuota,
 		"cpu_period", hostConfig.CPUPeriod)

drivers/docker/driver_test.go

@@ -1014,25 +1014,55 @@ func TestDockerDriver_CreateContainerConfig_Logging(t *testing.T) {
 	cases := []struct {
 		name           string
 		loggingConfig  DockerLogging
-		expectedDriver string
+		expectedConfig DockerLogging
 	}{
 		{
 			"simple type",
 			DockerLogging{Type: "fluentd"},
-			"fluentd",
+			DockerLogging{
+				Type:   "fluentd",
+				Config: map[string]string{},
+			},
 		},
 		{
 			"simple driver",
 			DockerLogging{Driver: "fluentd"},
-			"fluentd",
+			DockerLogging{
+				Type:   "fluentd",
+				Config: map[string]string{},
+			},
 		},
 		{
 			"type takes precedence",
 			DockerLogging{
 				Type:   "json-file",
 				Driver: "fluentd",
 			},
-			"json-file",
+			DockerLogging{
+				Type:   "json-file",
+				Config: map[string]string{},
+			},
+		},
+		{
+			"user config takes precedence, even if no type provided",
+			DockerLogging{
+				Type:   "",
+				Config: map[string]string{"max-file": "3", "max-size": "10m"},
+			},
+			DockerLogging{
+				Type:   "",
+				Config: map[string]string{"max-file": "3", "max-size": "10m"},
+			},
+		},
+		{
+			"defaults to json-file w/ log rotation",
+			DockerLogging{
+				Type: "",
+			},
+			DockerLogging{
+				Type:   "json-file",
+				Config: map[string]string{"max-file": "2", "max-size": "2m"},
+			},
 		},
 	}
 
@@ -1049,7 +1079,9 @@ func TestDockerDriver_CreateContainerConfig_Logging(t *testing.T) {
 			cc, err := driver.createContainerConfig(task, cfg, "org/repo:0.1")
 			require.NoError(t, err)
 
-			require.Equal(t, c.expectedDriver, cc.HostConfig.LogConfig.Type)
+			require.Equal(t, c.expectedConfig.Type, cc.HostConfig.LogConfig.Type)
+			require.Equal(t, c.expectedConfig.Config["max-file"], cc.HostConfig.LogConfig.Config["max-file"])
+			require.Equal(t, c.expectedConfig.Config["max-size"], cc.HostConfig.LogConfig.Config["max-size"])
 		})
 	}
 }

website/source/docs/drivers/docker.html.md

@@ -166,8 +166,8 @@ The `docker` driver supports the following configuration in the job spec.  Only
     }
     ```
 
-* `logging` - (Optional) A key-value map of Docker logging options. The default
-  value is `syslog`.
+* `logging` - (Optional) A key-value map of Docker logging options. 
+    Defaults to `json-file` with log rotation (`max-file=2` and `max-size=2m`).
 
     ```hcl
     config {