Fix case of variables in docker login.

src/common

@@ -23,7 +23,7 @@ docker_login() {
     # TODO: detect registry url
     mkdir -p "$HOME/.docker"
     echo "$DOCKER_LOGIN_FILE_TMPL" | \
-        sed -e "s|{{BASE64_UNAME_PW}}|$(printf '%s:%s' "$username" "$password" | base64)|g" \
+        sed -e "s|{{BASE64_UNAME_PW}}|$(printf '%s:%s' "$USERNAME" "$PASSWORD" | base64)|g" \
             -e "s|{{REGISTRY_URL}}|$login_name|g" \
         > "$HOME/.docker/config.json"
 }