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