Pin miniconda version to avoid breaking changes

[edwardjkim]

Description of changes:

This PR pins miniconda version to avoid breaking changes. See a related PR aws/sagemaker-scikit-learn-container#58 for an example where a rebuild of the image can bump the Python version.

aws/sagemaker-scikit-learn-container#58 uses ARG variables for version numbers, but since the 1.0-1 branch is no longer in active development and we expect only security patches and bug fixes to go into this branch, I opted for not using ARG variables and making only minimal changes.

Looking at https://repo.anaconda.com/miniconda/, Miniconda3-latest-Linux-x86_64.sh was released on 2020-06-16 with checksum d63adf39f2c220950a063e0529d4ff74. Our last 1.0-1 deployment was on 2020-07-20. This corresponds to Miniconda3-py38_4.8.3-Linux-x86_64.sh.

The current prod image has Python 3.6. (The Python version is specified in here.

$ docker run 246618743249.dkr.ecr.us-west-2.amazonaws.com/sagemaker-xgboost:1.0-1-cpu-py3 sh -c 'python --version'
Python 3.6.10 :: Anaconda, Inc.

The image built from this PR:

$ docker run preprod-xgboost-container:1.0-1-cpu-py3 sh -c 'python --version'
Python 3.6.10 :: Anaconda, Inc.

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