feat: allow custom MaaS download base URL

maas-images.inc

@@ -37,12 +37,17 @@ function file_url()
 {
     local release_version="$1"
     local filename="$2"
+    local baseurl="https://images.maas.io/ephemeral-v3/stable/"
+
     if [ ! -f "$streams" ]; then
         return 1
     fi
     url=$(jq -r '.products[]|select(.version=="'"$OS_VERSION"'" and .arch=="amd64" and .kflavor=="generic" and .subarch=="ga-'"$OS_VERSION"'").versions."'"$release_version"'".items."'"$filename"'".path' \
         "$streams")
-    url="https://images.maas.io/ephemeral-v3/stable/$url"
+    if [ -n "$MAAS_DOWNLOAD_URL" ]; then
+        baseurl="$MAAS_DOWNLOAD_URL"
+    fi
+    url="$baseurl/$url"
 }
 
 function download_and_verify()