Use exec form of ENTRYPOINT for docker server (#1690)
* Use exec form of ENTRYPOINT for docker server
# Issue
The entrypoint currently uses the shell form - this prevents users from passing in any cmdline arguments... also passing a model_path in means the server only works in the envvar is set... however this is not what the error message says!
```
$ docker run -v /home/rakelkar/try/onnxzoo/style:/mnt/models -it mcr.microsoft.com/onnxruntime/server --model_path /mnt/models/model.onnx
Version: local_build
Commit ID: default
model_path must be the location of a valid file
Allowed options:
-h [ --help ] Shows a help message and exits
--log_level arg (=info) Logging level. Allowed options (case sensitive):
verbose, info, warning, error, fatal
--model_path arg Path to ONNX model
--address arg (=0.0.0.0) The base HTTP address
--http_port arg (=8001) HTTP port to listen to requests
--num_http_threads arg (=4) Number of http threads
--grpc_port arg (=50051) GRPC port to listen to requests
```
# Fix
1. remove the env var
2. use the exec form
* Update readme to use model_path arg
