ElastAlert will not support Python2.x should be update Dockerfile

[yichingwu]

As I know that elastalert not supported python 2.x in the new version, if possible please help to update the dockerfile with python 3.6.x version.
Python 2 NOT anymore supported from version 0.2.0

[yichingwu]

I saw there was a pr from @nqkdev , thank you.
#117

[wildgarden]

The fork seems to work. However I ran into following error:

gyp ERR! find Python Python is not set from command line or npm configuration...

The roort cause is nodejs/node-gyp#1268 . Sadly, gyp still relies on python2.

[jdeglopper]

I have submitted #122 which uses python2 for node-gyp but runs elastalert under python3. It appears to connect correctly to my ElasticSearch 7.2.0 cluster.