Added ability for syntax highlight in code blocks.

Docutils can leverage pygments python package to create syntax highlight
inside code blocks. Implementation consists two custom classes for
getting HTML code tag contain appropriate class keyword and value.

Dockerfile

@@ -1,4 +1,4 @@
 FROM gitea/gitea:latest
 COPY ./rst2htmlbody.py /bin/rst2htmlbody
-RUN apk --no-cache add py3-docutils && \
+RUN apk --no-cache add py3-docutils py3-pygments && \
     chmod 755 /bin/rst2htmlbody

rst2htmlbody.py

@@ -5,6 +5,20 @@ from docutils import core
 from docutils.writers import html5_polyglot
 
 
+# add class "chroma" to the code block, to get the syntax highlighting
+class CustomHTMLTranslator(html5_polyglot.HTMLTranslator):
+    def visit_literal_block(self, node):
+        self.body.append(self.starttag(node, 'pre', '', CLASS='literal-block'))
+        if 'code' in node.get('classes', []):
+            self.body.append('<code class="chroma">')
+
+
+class CustomWriter(html5_polyglot.Writer):
+    def __init__(self):
+        super().__init__()
+        self.translator_class = CustomHTMLTranslator
+
+
 def main():
     if not sys.stdin.isatty():
         text = sys.stdin.readlines()
@@ -15,8 +29,15 @@ def main():
         with open(sys.argv[1]) as fobj:
             text = fobj.read()
 
-    parts = core.publish_parts(writer=html5_polyglot.Writer(),
-                               source=''.join(text))
+    try:
+        import pygments
+        settings = {'syntax_highlight': 'short'}
+    except ImportError:
+        settings = {'syntax_highlight': 'none'}
+
+    parts = core.publish_parts(writer=CustomWriter(),
+                               source=''.join(text),
+                               settings_overrides=settings)
 
     print(parts.get('body', ''))