fix: restore CI for Traefik v1.7

Co-authored-by: Michael <michael.matur@gmail.com>

script/docs-verify-docker-image/Dockerfile

@@ -1,16 +1,37 @@
-FROM alpine:3.8
+FROM alpine:3.14 as alpine
 
 RUN apk --no-cache --no-progress add \
-    ca-certificates \
-    curl \
-    findutils \
+    libcurl \
+    ruby \
     ruby-bigdecimal \
     ruby-etc \
     ruby-ffi \
     ruby-json \
-    ruby-nokogiri=1.8.3-r0 \
-    tini \
-  && gem install --no-document html-proofer -v 3.9.3
+    ruby-nokogiri \
+    ruby-dev \
+    build-base
+
+RUN gem install html-proofer --version 3.19.0 --no-document -- --use-system-libraries
+
+# After Ruby, some NodeJS YAY!
+RUN apk --no-cache --no-progress add \
+    git \
+    nodejs \
+    npm
+
+# To handle 'not get uid/gid'
+RUN npm config set unsafe-perm true
+
+RUN npm install --global \
+    markdownlint@0.22.0 \
+    markdownlint-cli@0.26.0
+
+# Finally the shell tools we need for later
+# tini helps to terminate properly all the parallelized tasks when sending CTRL-C
+RUN apk --no-cache --no-progress add \
+    ca-certificates \
+    curl \
+    tini
 
 COPY ./validate.sh /validate.sh