feat: Send commands to weed shell from the docker image.

Add the ability to send commands to weed shell from the docker image.

Allows an operator to perform maintenance commands like so:
```
docker run \
  --rm \
  -e SHELL_FILER=localhost:8888 \
  -e SHELL_MASTER=localhost:9333 \
  chrislusf/seaweedfs:local \
  "shell" \
  "fs.configure -locationPrefix=/buckets/foo -volumeGrowthCount=3 -replication=002 -apply"
```

docker/entrypoint.sh

@@ -57,6 +57,12 @@ case "$1" in
   	exec /usr/bin/weed -logtostderr=true s3 $ARGS $@
 	;;
 
+  'shell')
+  	ARGS="-cluster=$SHELL_CLUSTER -filer=$SHELL_FILER -filerGroup=$SHELL_FILER_GROUP -master=$SHELL_MASTER -options=$SHELL_OPTIONS"
+  	shift
+  	exec echo "$@" | /usr/bin/weed -logtostderr=true shell $ARGS
+  ;;
+
   *)
   	exec /usr/bin/weed $@
 	;;