Add support for custom `caddyfile` #104

Job	Run time
Build and Run Alpine-based Docker image (8.2)	5m 28s
Build and Run Debian-based Docker image (8.2)	3m 25s
Build and Run Alpine-based Docker image (8.3)	5m 20s
Build and Run Debian-based Docker image (8.3)	3m 23s
	17m 36s

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