Update docker build to use new Poetry installer

[glennmatthews]

Proposed Changes

As seen in Docker build:

#8 [dev  3/14] RUN curl -sSL https://raw.githubusercontent.com/python-poetry/poetry/master/get-poetry.py -o /tmp/get-poetry.py &&     python /tmp/get-poetry.py &&     rm -f /tmp/get-poetry.py
#8 sha256:9ca25b50606107a3c6c57449104b10922ee10343f446931c55cd3bf241593322
#8 0.713 Retrieving Poetry metadata
#8 0.853 
#8 0.857 This installer is deprecated. Poetry versions installed using this script will not be able to use 'self update' command to upgrade to 1.2.0a1 or later.

Justification

We should migrate from get-poetry.py to install-poetry.py in order to make use of newer versions of Poetry as they become available.

[glennmatthews]

Fixed in #1073.