Added Dockerfile for linter and linkchecker

.gitlab-ci.yml

 # Datei: .gitlab-ci.yml
 variables:
     GIT_STRATEGY: clone
-    TEST_IMAGE: webpage:${CI_COMMIT_REF_NAME}
-    TEST_NETWORK: testnet
+    TEST_IMAGE: webpage:base
+    LINTER_IMAGE: webpage:linter
+    LINKCHECKER_IMAGE: webpage:linkchecker
 
 stages:
     - build
@@ -13,5 +14,8 @@ build:
         - pwd
         - ls -l
         - 'echo "TEST_IMAGE: ${TEST_IMAGE}"'
-        - 'echo "TEST_NETWORK: ${TEST_NETWORK}"'
+        - 'echo "LINTER_IMAGE: ${LINTER_IMAGE}"'
+        - 'echo "LINKCHECKER_IMAGE: ${LINKCHECKER_IMAGE}"'
         - docker build -t ${TEST_IMAGE} .
+        - docker build -t ${LINTER_IMAGE} linter/
+        - docker build -t ${LINKCHECKER_IMAGE} linkchecker/

linkchecker/Dockerfile

+FROM webpage:base
+
+RUN apt update && apt install -y nodejs npm
+
+RUN npm install -g markdown-link-check
+
+WORKDIR /src/linkchecker
+

linter/Dockerfile

+FROM webpage:base
+
+RUN apt update && apt install -y nodejs npm
+
+RUN npm install -g markdownlint-cli
+
+WORKDIR /src/linter
+
+ENTRYPOINT ["/usr/local/bin/markdownlint"]
+
+CMD ["--help"]
+

linter/config.json

+{
+    "default": false,
+    "first-heading-h1": true,
+    "header-increment": true,
+    "heading-style": { "style": "atx" },
+    "no-missing-space-atx": true,
+    "no-multiple-space-atx": true,
+    "blanks-around-headings": true,
+    "no-emphasis-as-heading": true,
+    "first-line-heading": true,
+    "single-title": true,
+    "no-empty-links": true,
+    "no-alt-text": true,
+    "no-multiple-blanks": true,
+    "single-trailing-newline": true,
+    "blanks-around-fences": true,
+    "blanks-around-lists": true,
+    "commands-show-output": true,
+    "line-length": { "line_length": 100, "code_blocks": false, "tables": false},
+    "no-missing-space-atx": true,
+    "no-multiple-space-atx": true
+}