add kramdown-parser-gfm and do some optimization

Dockerfile

@@ -2,10 +2,9 @@ FROM alpine:3.13
 
 LABEL org.opencontainers.image.authors="hanjo@bunix.de"
 
-ENV dev_packages="ca-certificates build-base libffi-dev ruby-dev ruby-rdoc ruby-irb gcc make musl-dev"
+ENV DEV_PACKAGES="build-base libffi-dev ruby-dev ruby-rdoc ruby-irb gcc make musl-dev"
 
-RUN apk add --no-cache ruby ruby-json libffi libstdc++ ${dev_packages} \
+RUN apk add --no-cache ca-certificates ruby ruby-json libffi libstdc++ ${DEV_PACKAGES}
-    && rm -rf /var/cache/apk/*
 
 ARG JEKYLL_VERSION
 RUN gem install jekyll -v ${JEKYLL_VERSION} \
@@ -13,6 +12,8 @@ RUN gem install jekyll -v ${JEKYLL_VERSION} \
     && gem install jekyll-auto-image \
     && gem install jekyll-tidy \
     && gem install jekyll-sitemap \
-    && gem install bigdecimal
+    && gem install bigdecimal \
+    && gem install kramdown-parser-gfm
 
-RUN apk del ${dev_packages}
+RUN apk del ${DEV_PACKAGES} \
+    && rm -rf /var/cache/apk/*