update docker setup (#601)

.devcontainer/Dockerfile

@@ -3,7 +3,7 @@
 # Licensed under the MIT License. See https://go.microsoft.com/fwlink/?linkid=2090316 for license information.
 #-------------------------------------------------------------------------------------------------------------
 
-FROM mcr.microsoft.com/vscode/devcontainers/python:0-3.6
+FROM mcr.microsoft.com/vscode/devcontainers/python:0-3.8
 
 #
 # Update the OS and maybe install packages
@@ -20,4 +20,4 @@ ENV DEBIAN_FRONTEND=dialog
 #
 # Install extras for development
 #
-RUN pip3 --disable-pip-version-check --no-cache-dir install hummingbird-ml[docs,tests,extra] autopep8
+RUN pip3 --disable-pip-version-check --no-cache-dir install hummingbird-ml[docs,tests,extra,onnx] autopep8

Dockerfile

@@ -1,5 +1,5 @@
 # basic setup
-FROM python:3.6
+FROM python:3.8
 RUN apt-get update && apt-get -y update
 RUN apt-get install -y sudo git