Fix shell check issues in alpine:

- make script POSIX compatible
- fix quoting

util/alpine.sh

@@ -2,14 +2,14 @@
 
 . ./common.sh
 
-MIRROR=http://dl-cdn.alpinelinux.org/alpine/
+MIRROR="http://dl-cdn.alpinelinux.org/alpine/"
 
 alp() {
-    local VER=$1
-    local REPOS=$2
+    alp_version="$1"
+    alp_repositories="$2"
 
-    for REPO in $REPOS; do
-        index alpine --sys alpine-$VER --mirror ${MIRROR}v$VER --repo $REPO
+    for repository in $alp_repositories; do
+        index alpine --sys alpine-"$alp_version" --mirror ${MIRROR}v"$alp_version" --repo "$repository"
     done
 }