Add exiftool to Dockerfile

This is required by workhorse to process images:

workhorse_1      | time="2020-07-15T11:49:31Z" level=info msg="running exiftool to remove any metadata" correlation_id=RvHSoAhBrK1 filename="WhatsApp Image 2019-10-09 at 15.40.41.jpeg"
workhorse_1      | time="2020-07-15T11:49:31Z" level=error msg=error correlation_id=RvHSoAhBrK1 error="handleFileUploads: extract files from multipart: start [exiftool -all= --IPTC:all --XMP-iptcExt:all -tagsFromFile @ -ResolutionUnit -XResolution -YResolution -YCbCrSubSampling -YCbCrPositioning -BitsPerSample -ImageHeight -ImageWidth -ImageSize -Copyright -CopyrightNotice -Orientation -]: exec: \"exiftool\": executable file not found in $PATH" method=POST uri=/h5bp/html5-boilerplate/uploads

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