ImageMagick update breaks PDF thumbnailing.

[benanhalt]

The following error started occurring:

convert: not authorized `/home/specify/attachments/bishopmuseum/originals/f8473d71-1e4d-4b85-9e15-963e5c2a4c12.pdf' @ error/constitute.c/ReadImage/412.
convert: unable to open image `white': No such file or directory @ error/blob.c/OpenBlob/2712.
convert: unable to open image `white': No such file or directory @ error/blob.c/OpenBlob/2712.
convert: no decode delegate for this image format `' @ error/constitute.c/ReadImage/501.
convert: no images defined `/home/specify/attachments/bishopmuseum/thumbnails/f8473d71-1e4d-4b85-9e15-963e5c2a4c12.png' @ error/convert.c/ConvertImageCommand/3210.

[benanhalt]

See: https://stackoverflow.com/a/52661288

Changing the PDF policy to "read" in /etc/ImageMagick-6/policy.xml seems sufficient:
<policy domain="coder" rights="read" pattern="PDF" />

[benanhalt]

This fix needs to be added to the docker image.

[grantfitzsimmons]

@melton-jason Can you make this change in the default image in Docker?