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"
```