ci: use locally built gem file for testing Docker image build

docker/ci/Dockerfile

@@ -0,0 +1,22 @@
+FROM alpine:3.13.5
+
+EXPOSE 5353 8080
+
+COPY pkg/dyndnsd-*.gem /tmp/dyndnsd.gem
+
+RUN apk --no-cache add openssl ca-certificates && \
+    apk --no-cache add ruby ruby-etc ruby-io-console ruby-json ruby-webrick && \
+    apk --no-cache add --virtual .build-deps linux-headers ruby-dev build-base tzdata && \
+    gem install --no-document /tmp/dyndnsd.gem && \
+    rm -rf /usr/lib/ruby/gems/*/cache/ && \
+    # set timezone to Berlin
+    cp /usr/share/zoneinfo/Europe/Berlin /etc/localtime && \
+    apk del .build-deps
+
+# Follow the principle of least privilege: run as unprivileged user.
+# Running as non-root enables running this image in platforms like OpenShift
+# that do not allow images running as root.
+# User ID 65534 is usually user 'nobody'.
+USER 65534
+
+ENTRYPOINT ["dyndnsd", "/etc/dyndnsd/config.yml"]