[#687] Adjust image pull policy inference (#689)

**Pull Request Checklist**
- [x] Fixes #687
- [ ] Tests added
- [x] Documentation/examples added
- [x] [Good commit messages](https://cbea.ms/git-commit/) and/or PR
title

**Description of PR**

This PR adds a small mapping of possible user inputs to map to
`ImagePullPolicy` objects as a QOL improvement/bug fix

---------

Signed-off-by: Flaviu Vadan <[email protected]>

src/hera/workflows/_mixins.py

@@ -77,7 +77,6 @@
     from hera.workflows.steps import Step
     from hera.workflows.task import Task
 
-
 _yaml: Optional[ModuleType] = None
 try:
     import yaml
@@ -176,12 +175,28 @@ class ContainerMixin(BaseMixin):
     termination_message_policy: Optional[TerminationMessagePolicy] = None
     tty: Optional[bool] = None
 
-    def _build_image_pull_policy(self) -> Optional[str]:
+    def _build_image_pull_policy(self) -> Optional[ImagePullPolicy]:
         if self.image_pull_policy is None:
             return None
         elif isinstance(self.image_pull_policy, ImagePullPolicy):
-            return self.image_pull_policy.value
-        return ImagePullPolicy[self.image_pull_policy.lower()].value
+            return self.image_pull_policy
+
+        # this helps map image pull policy values as a convenience
+        policy_mapper = {
+            # the following 2 are "normal" entries
+            **{ipp.name: ipp for ipp in ImagePullPolicy},
+            **{ipp.value: ipp for ipp in ImagePullPolicy},
+            # some users might submit the policy without underscores
+            **{ipp.value.lower().replace("_", ""): ipp for ipp in ImagePullPolicy},
+            # some users might submit the policy in lowercase
+            **{ipp.name.lower(): ipp for ipp in ImagePullPolicy},
+        }
+        try:
+            return ImagePullPolicy[policy_mapper[self.image_pull_policy].name]
+        except KeyError as e:
+            raise KeyError(
+                f"Supplied image policy {self.image_pull_policy} is not valid. Use one of {ImagePullPolicy.__members__}"
+            ) from e
 
     @validator("image", pre=True, always=True)
     def _set_image(cls, v):

src/hera/workflows/script.py

@@ -194,6 +194,8 @@ def _build_template(self) -> _ModelTemplate:
 
     def _build_script(self) -> _ModelScriptTemplate:
         assert isinstance(self.constructor, ScriptConstructor)
+        image_pull_policy = self._build_image_pull_policy()
+
         return self.constructor.transform_script_template_post_build(
             self,
             _ModelScriptTemplate(
@@ -202,7 +204,8 @@ def _build_script(self) -> _ModelScriptTemplate:
                 env=self._build_env(),
                 env_from=self._build_env_from(),
                 image=self.image,
-                image_pull_policy=self._build_image_pull_policy(),
+                # `image_pull_policy` in script wants a string not an `ImagePullPolicy` object
+                image_pull_policy=None if image_pull_policy is None else image_pull_policy.value,
                 lifecycle=self.lifecycle,
                 liveness_probe=self.liveness_probe,
                 name=self.container_name,