Update Alpine to 3.7 (for multiarch support)

This also adds an appropriate "Architectures" value to "generate-stackbrew-library.sh".

Signed-off-by: Konstantin Pavlov <thresh@nginx.com>

generate-stackbrew-library.sh

@@ -91,11 +91,10 @@ for version in "${versions[@]}"; do
 		variantAliases=( "${versionAliases[@]/%/-$variant}" )
 		variantAliases=( "${variantAliases[@]//latest-/}" )
 
-		# TODO Architectures once https://github.com/gliderlabs/docker-alpine/issues/304 is resolved
-
 		echo
 		cat <<-EOE
 			Tags: $(join ', ' "${variantAliases[@]}")
+			Architectures: amd64, arm32v6, arm64v8, i386, ppc64le, s390x
 			GitCommit: $commit
 			Directory: $version/$variant
 		EOE

mainline/alpine-perl/Dockerfile

-FROM alpine:3.5
+FROM alpine:3.7
 
 LABEL maintainer="NGINX Docker Maintainers <docker-maint@nginx.com>"
 

mainline/alpine/Dockerfile

-FROM alpine:3.5
+FROM alpine:3.7
 
 LABEL maintainer="NGINX Docker Maintainers <docker-maint@nginx.com>"
 

stable/alpine-perl/Dockerfile

-FROM alpine:3.5
+FROM alpine:3.7
 
 LABEL maintainer="NGINX Docker Maintainers <docker-maint@nginx.com>"
 

stable/alpine/Dockerfile

-FROM alpine:3.5
+FROM alpine:3.7
 
 LABEL maintainer="NGINX Docker Maintainers <docker-maint@nginx.com>"