Add scripts to generate the directory structure

Dockerfile

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+FROM alpine:3.9
+RUN apk add --no-cache lua5.3 lua-filesystem lua-lyaml lua-http
+COPY fetch-latest-releases.lua /usr/local/bin
+VOLUME /out
+ENTRYPOINT [ "/usr/local/bin/fetch-latest-releases.lua" ]

fetch-latest-releases.lua

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+#!/usr/bin/lua5.3
+
+-- script to fetch and parse latest-releases.yaml from master site
+-- and fetch the latest minitootfs images for all available branches
+
+local request = require("http.request")
+local cqueues = require("cqueues")
+local yaml = require("lyaml")
+local lfs = require("lfs")
+
+local mirror = "https://cz.alpinelinux.org/alpine"
+
+
+function fetch(url)
+	local headers, stream = assert(request.new_from_uri(url):go())
+	local body= assert(stream:get_body_as_string())
+	return headers:get(":status"), body
+end
+
+function errormsg(msg)
+	io.stderr:write(string.format("Error: %s: %s\n", errmsg, url))
+	return nil, msg
+end
+
+function fetch_file(url, outfile)
+	local headers, stream = assert(request.new_from_uri(url):go())
+	local partfile = string.format("%s.part", outfile)
+	local f, errmsg = io.open(partfile, "w")
+	if not f then
+		return errormsg(errmsg)
+	end
+	local ok, errmsg, errnum = stream:save_body_to_file(f)
+	f:close()
+	if not ok then
+		return errormsg(errmsg)
+	end
+	return os.rename(partfile, outfile)
+end
+
+
+function get_minirootfs(images, destdir)
+	for _,img in pairs(images) do
+		if img.flavor == "alpine-minirootfs" then
+			if destdir then
+				local url = string.format("%s/%s/releases/%s/%s",
+					mirror, img.branch, img.arch, img.file)
+				local archdir = string.format("%s/%s", destdir, img.arch)
+				local ok, errmsg = lfs.mkdir(archdir)
+				if not ok then
+					return errormsg(errmsg)
+				end
+				fetch_file(url, string.format("%s/%s", archdir, img.file))
+				print(img.file)
+			end
+			return { version=img.version, file=img.file, sha512=img.sha512 }
+		end
+	end
+end
+
+
+-- get array of minirootsfs releases --
+function get_releases(branch, destdir)
+	local arches = { "aarch64", "armhf", "armv7", "ppc64le" , "s390x", "x86", "x86_64" }
+	local t = {}
+	local loop = cqueues.new()
+	for _, arch in pairs(arches) do
+		loop:wrap(function()
+			local url = string.format("%s/%s/releases/%s/latest-releases.yaml",
+				mirror, branch, arch)
+			local status, body = fetch(url)
+			if status == "200" then
+				t[arch] = get_minirootfs(yaml.load(body), destdir)
+			end
+		end)
+	end
+	loop:loop()
+	return t
+end
+
+local branch = arg[1] or "edge"
+local destdir = arg[2]
+
+local f
+
+if destdir then
+	lfs.mkdir(destdir)
+	f = io.open(string.format("%s/checksums.sha512", destdir), "w")
+else
+	f = io.stdout
+end
+
+for arch,rel in pairs(get_releases(branch, destdir)) do
+	local line = string.format("%s  %s/%s\n", rel.sha512, arch, rel.file)
+	f:write(line)
+end
+
+if f ~= io.stdout then
+	f:close()
+end
+

prepare-branch.sh

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+#!/bin/sh -e
+
+branch=${1:-edge}
+
+dir=$(mktemp -d /tmp/docker-brew-alpine-XXXXXX)
+docker build -t docker-brew-alpine-fetch .
+docker run --user $(id -u) --rm -it -v $dir:/out docker-brew-alpine-fetch $branch /out
+
+(
+	cd $dir
+	sha512sum -c checksums.sha512
+
+	for img in */*.tar.gz; do
+		arch=${img%%/*}
+		rootfs=${img##*/}
+		cat > $arch/Dockerfile <<-EOF
+			FROM scratch
+			ADD ${rootfs} /
+			CMD ["/bin/sh"]
+		EOF
+	done
+)
+
+echo "COMPLETED"
+echo "=> Temp dir: $dir"
+