fix(digest): Add support for GitHub's docker registry

Github registry doesn't support digest yet, we need to download the manifest
and calculate the digest manually

Also fixing a few other issues:
 * Multi-stage dockerfiles override /usr/local/lib, moved jq and reg to /kaniko instead
 * The digest was fetched for the current tag, which doesn't exist yet. Fetching digest for the latest tag instead

Dockerfile

@@ -6,13 +6,12 @@ FROM gcr.io/kaniko-project/executor:debug
 
 SHELL ["/busybox/sh", "-c"]
 
-RUN mkdir -p /usr/local/bin && \
-    wget -O /usr/local/bin/jq \
+RUN wget -O /kaniko/jq \
     https://github.com/stedolan/jq/releases/download/jq-1.6/jq-linux64 && \
-    chmod +x /usr/local/bin/jq && \
-    wget -O /usr/local/bin/reg \
+    chmod +x /kaniko/jq && \
+    wget -O /kaniko/reg \
     https://github.com/genuinetools/reg/releases/download/v0.16.1/reg-linux-386 && \
-    chmod +x /usr/local/bin/reg
+    chmod +x /kaniko/reg
 
 COPY entrypoint.sh /
 COPY --from=certs /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt