Explicitly add ca-certificates to the release image (#258)

It occurs to me that maybe the firehose issue we're seeing is a
lack of certs sitting inside the release image. If this commit
finds its way to the master branch this was the case.

Signed-off-by: Brian L. Troutwine <blt@postmates.com>

docker/release/Dockerfile

@@ -2,13 +2,16 @@ FROM alpine:edge
 
 RUN echo '@testing http://dl-cdn.alpinelinux.org/alpine/edge/testing' >> /etc/apk/repositories
 
-RUN apk add --no-cache --update \
-  llvm-libunwind@testing \ 
-  openssl
-
 RUN apk update \
   && apk upgrade --no-cache
 
+RUN apk add --no-cache --update \
+  ca-certificates \
+  llvm-libunwind@testing \ 
+  openssl && \
+  update-ca-certificates && \
+  rm -rf /var/cache/apk/*
+
 ARG DIST_ROOT="target/bindir"
 
 COPY ${DIST_ROOT}/confd /etc/confd