Add environment variables

Environment-Variables.md

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+# Environment Variables
+You can use environment variables instead of arguments for `weed`.  
+For example:  
+instead of `weed master -port 5000 -mdir /tmp -volumePreallocate` you can use
+```shell
+export PORT=5000
+export MDIR=/tmp
+export VOLUMEPREALLOCATE=true # or export VOLUMEPREALLOCATE=
+weed master
+```
+
+# Docker
+This is useful for using docker and docker compose  
+You have to override entrypoint to `weed` because defautl [entrypoint](https://github.com/seaweedfs/seaweedfs/blob/master/docker/entrypoint.sh) use default values for `volumeSizeLimitMB`, `volumePreallocate`, `mdir`, `dir`, `max` argumants and setting environment variables won't change these values.
+## Dcoker
+```shell
+docker run --entrypoint weed -it -e MDIR=/tmp -e PORT=5000 -e VOLUMEPREALLOCATE=true chrislusf/seaweedfs:3.45 master
+```
+
+## Docker Compose
+```yaml
+version: '3.9'
+services:
+  master:
+    image: chrislusf/seaweedfs:3.45
+    ports:
+      - 9333:9333
+      - 19333:19333
+    environment:
+      MDIR: /data
+      PORT: 9333
+      VOLUMEPREALLOCATE: 'true'
+      # or `VOLUMEPREALLOCATE:`
+    entrypoint: weed
+    command: master 
+```