Use own mirror for clones

This patch replaces the official location with the local github mirror,
in order to prevent things from disappearing.

Dockerfile

@@ -4,7 +4,7 @@ ARG VERSION=master
 
 RUN true \
     && apk add --no-cache --virtual .build-utils git make \
-    && git clone https://github.com/m13253/dns-over-https.git --depth 1 -b "$VERSION" \
+    && git clone https://git.shivering-isles.com/github-mirror/m13253/dns-over-https.git --depth 1 -b "$VERSION" \
     && cd dns-over-https \
     && make \
     && make install \