In order to run multiple commands in docker, use /bin/bash -c with a semicolon ;
docker run image /bin/bash -c "cd /some/path; python a.py"
In this case the second command (python) will be executed only if the first command (cd) returns no error or an exit status. To avoid this use && instead of ; (semi-colon)
docker run image /bin/bash -c "cd /some/path && python a.py"
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