template-instead-of-copy: reduce number of false positives

- adds extra conditions for triggering the rule, basically eliminating
  the most simple strings from triggering it
- update documentation

Fixes: #2501

src/ansiblelint/rules/template_instead_of_copy.md

@@ -0,0 +1,67 @@
+# template-instead-of-copy
+
+This rule identifies the presence of variable interpolation inside `copy`
+module and recommends instead the use of the `template` module. This is based on
+the official [recommendation](https://docs.ansible.com/ansible/latest/collections/ansible/builtin/copy_module.html#synopsis)
+from Ansible documentation.
+
+This rule will be triggered if any of the following conditions are met:
+
+- `content` is not a string
+- `content` contains any Unicode character (not being pure ASCII)
+- `content` is multiline
+- `content` contains at least one of `{{`, `{%`, `{#` substrings
+
+```{warning}
+Unless you explicitly include a `\n` at the end of your string, the final
+file will not have a trailing newline. This is a very common error, as many
+tools fail to work properly if the last line is not terminated by a newline.
+```
+
+## Problematic Code
+
+```yaml
+---
+- name: Example playbook
+  hosts: localhost
+  tasks:
+    - name: Write file content
+      ansible.builtin.copy:
+        content: { "foo": "bar" } # <-- should use template instead
+        dest: /tmp/foo.txt
+```
+
+Please note that in this case, the content of the file will be
+`{ "foo", "bar" }`, which is different from the original content (see that
+the single quotation marks were modified by Jinja into double ones). The file
+will also not have an ending newline, something that is usually expected by
+people knowing Python language.
+
+Several other conversions can happen while using `copy`, including up to 4
+levels of templating. On the other hand, the `template` module will only
+perform 1 level of templating.
+
+## Correct Code
+
+```yaml
+---
+- name: Example playbook (1st solution, using `template` module)
+  hosts: localhost
+  tasks:
+    - name: Write file content
+      ansible.builtin.template:
+        # Add `templates/foo.txt.j2` with what you need inside
+        src: foo.txt.j2
+        dest: /tmp/foo.txt
+```
+
+```yaml
+---
+- name: Example playbook (2nd solution, use only a very simple string)
+  hosts: localhost
+  tasks:
+    - name: Write file content
+      ansible.builtin.copy:
+        content: "{ 'foo': 'bar' }"
+        dest: /tmp/foo.txt
+```

src/ansiblelint/rules/template_instead_of_copy.py

@@ -34,7 +34,14 @@ def matchtask(
         """Confirm if current rule is matching a specific task."""
         if task["action"]["__ansible_module__"] == "copy":
             content = task["action"].get("content", "")
-            if "{{" in content:
+            if not isinstance(content, str):
+                return True
+            for seq in ("{{", "{#", "{%"):
+                if seq in content:
+                    return True
+            try:
+                content.encode("ascii")
+            except UnicodeEncodeError:
                 return True
         return False