Add support for custom `caddyfile` (#106) #105

Job	Run time
Build and Run Alpine-based Docker image (8.2)	5m 38s
Build and Run Debian-based Docker image (8.2)	3m 28s
Build and Run Alpine-based Docker image (8.3)	5m 55s
Build and Run Debian-based Docker image (8.3)	3m 26s
	18m 27s

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