Replace hardcoded python version with ARG variable The Dockerfile had a hardcoded Python version in install paths and COPY command in the second stage build. Replace that with a hardcoded ARG pythonversion. Add RUN command to verify pythonversion against the actual version installed in the docker image. If they do not match generate an appropriate --build-arg for the docker build command that will match the docker image version. Document the use of the variable and why it's needed in installation.txt. Also fix typos referencing --build-args and bad values for the arguments.

<!--
 This is the default body that is displayed when people visit the
 tracker. The tag below lists the currently open issues. You may
 replace it with a greeting message, or a different list of issues or
 whatever. It's a good idea to have the issues on the front page though
-->
<span tal:replace="structure python:db.bug.renderWith('index',
    sort=[('-', 'activity')], filter=['status'],
    columns=['activity','title','creator'],
    filterspec={'status':['1','2','4']})" />