Update dockerfile to modify policy.xml in-place

[TrippleCCC]

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it
• I have formatted my code using black -t py36

The last line in the Dockerfile currently removes all content from the policy.xml file since the file cannot be read from and simultaneously written too. This fix edits the file in place using sed -i.

This bug can be reproduced by building the docker image and then running docker run (tagname) cat /etc/ImageMagick-6/policy.xml

[coveralls]

Coverage remained the same at 67.544% when pulling 1c9eaaf on TrippleCCC:docker-bug into 79b2c1d on Zulko:master.

[tburrows13]

Thanks for spotting this. I'll merge it now, although I'm still hoping that we can get rid of ImageMagick entirely soon!