Specify exact version of `web3` in Dockerfile

[alex-murashkin]

Seems like Web3.py recently got updated and no longer supports Python 2.7 (compare https://pypi.org/project/web3/4.2.1/ to https://pypi.org/project/web3/3.16.5/ , the supports list)

However, the present Dockerfile seems to use Python 2.7

To work around that, could replace RUN pip install requests web3 with RUN pip install requests web3==3.16.5 to use the exact web3 version that is still compatible with Python 2.7

[lazzarello]

#336 should fix this, using the Ubuntu update-alternatives system. Waiting on review and merge.