docs: improve release process

docker/Dockerfile

@@ -2,7 +2,7 @@ FROM alpine:3.13.5
 
 EXPOSE 5353 8080
 
-ARG DYNDNSD_VERSION=3.4.1
+ARG DYNDNSD_VERSION
 
 RUN apk --no-cache add openssl ca-certificates && \
     apk --no-cache add ruby ruby-etc ruby-io-console ruby-json ruby-webrick && \

upgrade-version.sh

@@ -0,0 +1,13 @@
+#!/bin/bash -eux
+
+sed -i "s/$1/$2/g" lib/dyndnsd/version.rb
+
+release_date=$(LC_ALL=en_US.utf8 date +"%B %-d, %Y")
+
+if grep "## $2 (" CHANGELOG.md; then
+    true
+elif grep "## $2" CHANGELOG.md; then
+    sed -i "s/## $2/## $2 ($release_date)/g" CHANGELOG.md
+else
+    echo "## $2 ($release_date)" >> CHANGELOG.md
+fi