Filer startup fails when target directory doesn't exist

[Flowm]

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Describe the bug
With the official docker image (chrislusf/seaweedfs) configuring the filer with leveldb3 as filer store with target dir /data/filerldb3 fails on startup as the directory doesn't exist.

W0314 09:04:01     1 filer_server.go:117] skipping default store dir in ./filerldb2
I0314 09:04:01     1 leveldb3_store.go:47] filer store leveldb3 dir: /data/filerldb3
F0314 09:04:01     1 configuration.go:25] failed to initialize store for leveldb3: Check Level Folder /data/filerldb3 Writable: stat /data/filerldb3: no such file or directory
goroutine 1 [running]:
github.com/chrislusf/seaweedfs/weed/glog.stacks(0xc0005b7a00, 0xc0005b01e0, 0xb0, 0x1d7)
        /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/glog/glog.go:767 +0xb9
github.com/chrislusf/seaweedfs/weed/glog.(*loggingT).output(0x363eb40, 0xc000000003, 0xc00060a700, 0x2b487cc, 0x10, 0x19, 0x0)
        /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/glog/glog.go:718 +0x37a
github.com/chrislusf/seaweedfs/weed/glog.(*loggingT).printf(0x363eb40, 0x3, 0x22e8ae5, 0x26, 0xc00035f9b8, 0x2, 0x2)
        /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/glog/glog.go:656 +0x153
github.com/chrislusf/seaweedfs/weed/glog.Fatalf(...)
        /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/glog/glog.go:1149
github.com/chrislusf/seaweedfs/weed/filer.(*Filer).LoadConfiguration(0xc0006fe630, 0x3635f60)
        /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/filer/configuration.go:25 +0xf7e
github.com/chrislusf/seaweedfs/weed/server.NewFilerServer(0xc0003f9440, 0xc0003f9440, 0xc0000da180, 0x1, 0x0, 0x0)
        /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/server/filer_server.go:127 +0x593
github.com/chrislusf/seaweedfs/weed/command.(*FilerOptions).startFiler(0x363e8c0)
        /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/command/filer.go:159 +0x338
github.com/chrislusf/seaweedfs/weed/command.runFiler(0x361f1c0, 0xc00013c170, 0x0, 0x0, 0x0)
        /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/command/filer.go:138 +0x92
main.main()
        /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/weed.go:66 +0x30f

System Setup

• Output of weed version

version 30GB 2.26 ca74f64 linux amd64

• Minimal docker-compose.yml

version: '3.5'

services:
  master:
    image: chrislusf/seaweedfs
    command: "master -ip=master"

  filer:
    image: chrislusf/seaweedfs
    volumes:
      - ./filer.toml:/etc/seaweedfs/filer.toml
    command: 'filer -master="master:9333"'
    tty: true
    stdin_open: true
    depends_on:
      - master

• Contents of filer.toml

[leveldb3]
enabled = true
dir = "/data/filerldb3"

• List the command line to start:

# Minimal example with the two files from above in a single directory
$ ls
docker-compose.yml  filer.toml
$ docker-compose up
Starting example_master_1 ... done
Starting example_filer_1  ... done
Attaching to example_master_1, example_filer_1
master_1  | I0314 09:39:37     1 file_util.go:23] Folder /data Permission: -rwxr-xr-x
master_1  | I0314 09:39:37     1 master.go:168] current: master:9333 peers:
master_1  | I0314 09:39:37     1 master_server.go:107] Volume Size Limit is 1024 MB
master_1  | I0314 09:39:37     1 master_server.go:192] adminScripts:
master_1  | I0314 09:39:37     1 master.go:122] Start Seaweed Master 30GB 2.26 ca74f64 at 0.0.0.0:9333
master_1  | I0314 09:39:37     1 raft_server.go:70] Starting RaftServer with master:9333
master_1  | I0314 09:39:37     1 raft_server.go:129] current cluster leader:
master_1  | I0314 09:39:37     1 master.go:146] Start Seaweed Master 30GB 2.26 ca74f64 grpc server at 0.0.0.0:19333
master_1  | I0314 09:39:39     1 masterclient.go:78] No existing leader found!
master_1  | I0314 09:39:39     1 raft_server.go:154] Initializing new cluster
master_1  | I0314 09:39:39     1 master_server.go:141] leader change event:  => master:9333
master_1  | I0314 09:39:39     1 master_server.go:143] [ master:9333 ] master:9333 becomes leader.
master_1  | I0314 09:39:42     1 masterclient.go:126] redirected to leader master:9333
master_1  | I0314 09:39:42     1 master_grpc_server.go:245] + client master@172.24.0.2:55276
filer_1   | W0314 09:39:43     1 filer_server.go:117] skipping default store dir in ./filerldb2
master_1  | I0314 09:39:43     1 master_grpc_server.go:245] + client filer@172.24.0.3:8888
filer_1   | I0314 09:39:43     1 leveldb3_store.go:47] filer store leveldb3 dir: /data/filerldb3
filer_1   | F0314 09:39:43     1 configuration.go:25] failed to initialize store for leveldb3: Check Level Folder /data/filerldb3 Writable: stat /data/filerldb3: no such file or directory
filer_1   | goroutine 1 [running]:
filer_1   | github.com/chrislusf/seaweedfs/weed/glog.stacks(0xc0003d6000, 0xc0003d81e0, 0xb0, 0x1d7)
filer_1   |     /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/glog/glog.go:767 +0xb9
filer_1   | github.com/chrislusf/seaweedfs/weed/glog.(*loggingT).output(0x363eb40, 0xc000000003, 0xc000322700, 0x2b487cc, 0x10, 0x19, 0x0)
filer_1   |     /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/glog/glog.go:718 +0x37a
filer_1   | github.com/chrislusf/seaweedfs/weed/glog.(*loggingT).printf(0x363eb40, 0x3, 0x22e8ae5, 0x26, 0xc0006d19b8, 0x2, 0x2)
filer_1   |     /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/glog/glog.go:656 +0x153
filer_1   | github.com/chrislusf/seaweedfs/weed/glog.Fatalf(...)
filer_1   |     /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/glog/glog.go:1149
filer_1   | github.com/chrislusf/seaweedfs/weed/filer.(*Filer).LoadConfiguration(0xc0007173f0, 0x3635f60)
filer_1   |     /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/filer/configuration.go:25 +0xf7e
filer_1   | github.com/chrislusf/seaweedfs/weed/server.NewFilerServer(0xc000737580, 0xc000737580, 0xc0001f2cc0, 0x1, 0x0, 0x0)
filer_1   |     /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/server/filer_server.go:127 +0x593
filer_1   | github.com/chrislusf/seaweedfs/weed/command.(*FilerOptions).startFiler(0x363e8c0)
filer_1   |     /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/command/filer.go:159 +0x338
filer_1   | github.com/chrislusf/seaweedfs/weed/command.runFiler(0x361f1c0, 0xc000120170, 0x0, 0x0, 0x0)
filer_1   |     /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/command/filer.go:138 +0x92
filer_1   | main.main()
filer_1   |     /home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/weed.go:66 +0x30f
master_1  | I0314 09:39:43     1 master_grpc_server.go:261] - client filer@172.24.0.3:8888
example_filer_1 exited with code 255

Expected behavior
As the /data folder already exists in the docker contianer I would expect the filer to create the /data/filerldb3 directory on startup.

The default storage location /data/filerldb2 is already created in the Dockerfile, but there is no obvious way to create the additional required directory without creating a custom Dockerfile or overriding the entrypoint to do a mkdir first.