install python into containers for use during mapred

[leonbovett]

It would be really useful to have python installed in the containers, so that python scripts can be executed during invocation of mapred.

• Add a build step to all the containers (for consistency)
• Install python 3 in each container
• Ensure python 3 is the default in each container

[wxw-matt]

Thanks for the pull request. Will test it and merge it soon.

[Coomman]

merge please :)

[anuranjit]

./postgres.env file is missing here .