support for docker-ce

It seems that the custom-script-extension is not compatible with the docker-ce image. This image is used by the template provided at https://www.docker.com/docker-azure.

The error I see is:
```
/var/lib/waagent/Microsoft.Azure.Extensions.CustomScript-2.0.2/bin/custom-script-extension: No such file or directory
```
But this file does exist.
```
swarm-manager000000:/# ls -lh /var/lib/waagent/Microsoft.Azure.Extensions.CustomScript-2.0.2/bin/custom-script-extension
-rwxr-xr-x 1 root root 10.6M Apr 19 09:54 /var/lib/waagent/Microsoft.Azure.Extensions.CustomScript-2.0.2/bin/custom-script-extension
```

I suspect a conflict with custom-script-extension which is compiled against libc while it looks to me that docker-ce is based on alpine and musl.

The log files
[waagent.log.txt](https://github.com/Azure/custom-script-extension-linux/files/935428/waagent.log.txt)
[handler.log.txt](https://github.com/Azure/custom-script-extension-linux/files/935429/handler.log.txt)