Add a Gitlab job for verifying the Bash formatting

This is done using [shfmt](https://github.com/mvdan/sh), with the following
settings:

* Bash syntax
* 4 spaces indentation
* Require indented switch statements
* Keep column alignment padding
* Allow binary operators to start on a new line

Changelog: None
Signed-off-by: Ole Petter <ole.orhagen@northern.tech>

.gitlab-ci.yml

@@ -52,6 +52,53 @@ build:
     paths:
       - image.tar
 
+test:format:
+  stage: test
+  needs: []
+  image: alpine:3.13
+  before_script:
+    - apk update
+    - apk upgrade
+    - apk add --update bash git shfmt
+    - shfmt -version
+  script:
+    - /bin/bash
+    - SCRIPTS=$(find modules configs scripts -type f
+            -not -name "*.md"
+            -not -wholename "scripts/test/*")
+    # shfmt
+    ## Language=Bash
+    #     -ln bash
+    ## Indent=4 spaces
+    #     -i 4
+    ## List files who differ in formatting
+    #     -l
+    ## Redirect operators should be followed by a space
+    #     -sr
+    ## Indent switch statements
+    #     -ci
+    ## Keep column alignment padding
+    #     -kp
+    ## Binary operators like &&, || may start a new line
+    #     -bn
+    ## Overwrite the source files
+    #     -w
+    - |
+        shfmt \
+        -ln bash \
+        -i 4 \
+        -l \
+        -sr \
+        -ci \
+        -kp \
+        -bn \
+        -w \
+        ${SCRIPTS}
+      # Print diff
+    - git diff HEAD
+      # Actual test: exits with non zero status if diff
+    - git diff-index --quiet HEAD
+
 .template_convert_raspbian: &convert_raspbian
   stage: convert
   needs: