Refine docker-run command in README.md (#3098)

#### What type of PR is this?

/kind documentation

#### What this PR does / why we need it:

- Use arguments instead of environments.
- Change Docker image halo:2.0 into halo:2.1

#### Does this PR introduce a user-facing change?

```release-note
None
```

README.md

@@ -29,10 +29,10 @@ docker run \
   --name halo \
   -p 8090:8090 \
   -v ~/.halo2:/root/.halo2 \
-  -e HALO_EXTERNAL_URL=http://localhost:8090/ \
-  -e HALO_SECURITY_INITIALIZER_SUPERADMINUSERNAME=admin \
-  -e HALO_SECURITY_INITIALIZER_SUPERADMINPASSWORD=P@88w0rd \
-  halohub/halo:2.0
+  halohub/halo:2.1 \
+  --halo.external-url=http://localhost:8090/ \
+  --halo.security.initializer.superadminuser=admin \
+  --halo.security.initializer.superadminpassword=P@88w0rd
 ```
 
 以上仅作为体验使用，详细部署文档请查阅：<https://docs.halo.run/getting-started/install/docker-compose>