diff --git a/fetch-latest-releases.lua b/fetch-latest-releases.lua
index fc74dea..2aaa3cb 100755
--- a/fetch-latest-releases.lua
+++ b/fetch-latest-releases.lua
@@ -8,7 +8,7 @@ local cqueues = require("cqueues")
 local yaml = require("lyaml")
 local lfs = require("lfs")
 
-local mirror = "https://cz.alpinelinux.org/alpine"
+local mirror = os.getenv("MIRROR") or "https://cz.alpinelinux.org/alpine"
 
 function fatal(...)
 	errormsg(...)
diff --git a/prepare-branch.sh b/prepare-branch.sh
index f133a03..72b74e6 100755
--- a/prepare-branch.sh
+++ b/prepare-branch.sh
@@ -1,6 +1,5 @@
 #!/bin/sh -e
 
-
 run_tests() {
 	local branch="$1"
 	local dir="$2"
@@ -15,7 +14,11 @@ prepare() {
 	local branch="$1"
 	local dir=$(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
+	docker run \
+		${MIRROR+ -e "MIRROR=$MIRROR"} \
+		--user $(id -u) --rm -it \
+		-v $dir:/out \
+		docker-brew-alpine-fetch $branch /out
 	echo "=> Verifying checksums"
 	( cd $dir && sha512sum -c checksums.sha512)
 	echo "=> temp dir: $dir"