Move from the knot image to Alpine.

The builds have been broken for a while. Because the knot image is based
on Debian. Use Alpine instead (not because I'm sure the image will be
smaller or better, but it's less work to keep the existing Alpine
stuff).
Also, run as a limited user.

Dockerfile

-FROM cznic/knot-resolver
+FROM alpine:latest
 RUN echo '@testing http://dl-cdn.alpinelinux.org/alpine/edge/testing' >> /etc/apk/repositories && \
+    echo '@community http://dl-cdn.alpinelinux.org/alpine/edge/community' >> /etc/apk/repositories && \
     apk add --update --no-cache \
+        bash \
         dma@testing \
+        gosu@testing \
         iproute2 \
+        knot-resolver@community \
         knot-utils \
         mailx \
-        mtr
-COPY entrypoint /entrypoint
+        mtr \
+    && \
+    ln -s /usr/bin/kdig /usr/local/bin/dig && \
+    ln -s /usr/bin/khost /usr/local/bin/host
+COPY entrypoint /usr/local/sbin/entrypoint
 WORKDIR /
-ENTRYPOINT [ "/entrypoint" ]
+ENTRYPOINT [ "entrypoint" ]
 CMD [ "bash", "--login" ]

entrypoint

 #!/bin/sh
-set -eu
-echo "net = { '127.0.0.1', '::1'}" | nohup kresd -f1 &
+set -eux
+kresd --addr '127.0.0.1' --noninteractive &
 echo 'nameserver 127.0.0.1' > /etc/resolv.conf
-eval "exec $@"
+eval "exec gosu nobody:nobody $@"