Dockerfile: use folded block for rule description in docs front matter

Prevously, descriptions containing a colon would result in broken yaml
in the front matter of the generated docs.

Signed-off-by: David Karlsson <[email protected]>

Author: dvdksn

frontend/dockerfile/linter/generate.go

@@ -30,7 +30,8 @@ type Rule struct {
 
 const tmplStr = `---
 title: {{ .Rule.Name }}
-description: {{ .Rule.Description }}
+description: >-
+  {{ .Rule.Description }}
 {{- if .Rule.URLAlias }}
 aliases:
   - {{ .Rule.URLAlias }}