One use of $VER was forgotten.

Dockerfile

@@ -3,7 +3,7 @@ ARG VER
 RUN \
         mkdir -p /home/htmly/content && \
         cd /home/htmly && \
-        curl -o /tmp/htmly,${VER}.tar.gz https://codeload.github.com/danpros/htmly/tar.gz/v2.7.4 && \
+        curl -o /tmp/htmly,${VER}.tar.gz https://codeload.github.com/danpros/htmly/tar.gz/${VER} && \
         tar -x --strip-components=1 -zf /tmp/htmly,${VER}.tar.gz && \
         chown -R www-data:www-data . && \
         rm /tmp/htmly,${VER}.tar.gz