Update generate-stackbrew-library.sh to support BASHBREW_LIBRARY for easier cascading updates

See docker-library/official-images#17640 (comment)

Author: tianon

generate-stackbrew-library.sh

@@ -49,17 +49,19 @@ dirCommit() {
 
 getArches() {
 	local repo="$1"; shift
-	local officialImagesUrl='https://github.com/docker-library/official-images/raw/master/library/'
+	local officialImagesBase="${BASHBREW_LIBRARY:-https://github.com/docker-library/official-images/raw/HEAD/library}/"
 
-	eval "declare -g -A parentRepoToArches=( $(
-		find -name Dockerfile -exec awk '
+	local parentRepoToArchesStr
+	parentRepoToArchesStr="$(
+		find -name 'Dockerfile' -exec awk -v officialImagesBase="$officialImagesBase" '
 			/^FROM/ {
 				if (index($2, ":") > 0 && index($2, "'"$repo"'") == 0) {
-					print "'"$officialImagesUrl"'" $2
+					printf "%s%s\n", officialImagesBase, $2
 				}
 			}' '{}' + | sort -u \
-		| xargs bashbrew cat --format '[{{ .RepoName }}:{{ .TagName }}]="{{ join " " .TagEntry.Architectures }}"'
-	) )"
+		| xargs -r bashbrew cat --format '["{{ .RepoName }}:{{ .TagName }}"]="{{ join " " .TagEntry.Architectures }}"'
+	)"
+	eval "declare -g -A parentRepoToArches=( $parentRepoToArchesStr )"
 }
 getArches 'rabbitmq'