container stats: ignore context.Canceled errors

we often receive context.Canceled errors when a container exits during
docker stats collection.

there is a benign race condition where if we process the error first
before processing that the context is "Done", then we will log this
canceled error as a warning message here:
https://github.com/aws/amazon-ecs-agent/blob/5be7aa08bed215a557f48c16d8201ad3db59a9be/agent/stats/container.go#L118-L122

this change ignores these context.Canceled errors so that we don't log
them. This will eliminate log messages that look like this when a
container exits:

level=warn time=2020-12-25T07:51:33Z msg="Error encountered processing metrics stream from docker, this may affect cloudwatch metric accuracy: DockerGoClient: Unable to decode stats for container REDACTED: context canceled" module=container.go

agent/stats/container.go

@@ -118,8 +118,13 @@ func (container *StatsContainer) processStatsStream() error {
 		case <-container.ctx.Done():
 			return nil
 		case err := <-errC:
-			seelog.Warnf("Error encountered processing metrics stream from docker, this may affect cloudwatch metric accuracy: %s", err)
-			returnError = true
+			select {
+			case <-container.ctx.Done():
+				// ignore error when container.ctx.Done()
+			default:
+				seelog.Warnf("Error encountered processing metrics stream from docker, this may affect cloudwatch metric accuracy: %s", err)
+				returnError = true
+			}
 		case rawStat, ok := <-dockerStats:
 			if !ok {
 				if returnError {