Create and publish a Docker image

Add a utility to just run the thumbnailer (#633) #72

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Run time

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Job	Run time
build (linux/amd64)	5m 53s
build (linux/arm/v6)	49m 50s
build (linux/arm/v7)	49m 8s
build (linux/arm64)	48m 32s
merge	9s
	2h 33m 32s