Do not lowercase Docker image tags.

src/python/pants/backend/docker/goals/package_image.py

@@ -78,9 +78,7 @@ def format_tag(self, tag: str, interpolation_context: DockerInterpolationContext
         source = DockerInterpolationContext.TextSource(
             address=self.address, target_alias="docker_image", field_alias=self.tags.alias
         )
-        return interpolation_context.format(
-            tag, source=source, error_cls=DockerImageTagValueError
-        ).lower()
+        return interpolation_context.format(tag, source=source, error_cls=DockerImageTagValueError)
 
     def format_repository(
         self, default_repository: str, interpolation_context: DockerInterpolationContext

src/python/pants/backend/docker/goals/package_image_test.py

@@ -993,3 +993,28 @@ def test_get_context_root(
 )
 def test_parse_image_id_from_docker_build_output(expected: str, stdout: str, stderr: str) -> None:
     assert expected == parse_image_id_from_docker_build_output(stdout.encode(), stderr.encode())
+
+
+@pytest.mark.parametrize(
+    "raw_values, expect_raises, image_refs",
+    [
+        (dict(name="lowercase"), no_exception(), ("lowercase:latest",)),
+        (dict(name="CamelCase"), no_exception(), ("camelcase:latest",)),
+        (dict(image_tags=["CamelCase"]), no_exception(), ("image:CamelCase",)),
+        (dict(registries=["REG1.example.net"]), no_exception(), ("REG1.example.net/image:latest",)),
+    ],
+)
+def test_image_ref_formatting(
+    raw_values: dict, expect_raises: ContextManager, image_refs: tuple[str, ...]
+) -> None:
+    address = Address("test", target_name=raw_values.pop("name", "image"))
+    tgt = DockerImageTarget(raw_values, address)
+    field_set = DockerFieldSet.create(tgt)
+    default_repository = "{name}"
+    registries = DockerRegistries.from_dict({})
+    interpolation_context = DockerInterpolationContext.from_dict({})
+    with expect_raises:
+        assert (
+            field_set.image_refs(default_repository, registries, interpolation_context)
+            == image_refs
+        )

src/python/pants/testutil/pytest_util.py

@@ -31,11 +31,17 @@ def no_exception():
     When declaring parametrized tests, the test function can take a exceptions
     expectation as input, and always use a with-block for the code under test.
 
-        @pytest.mark.parametrize('answer, expect_raises', [
-            (42, no_exception()),
-            (12, pytest.raises(WrongAnswer)),
-        ])
-        def test_search_for_the_meaning_of_life_universe_and_everything(answer, expect_raises):
+        @pytest.mark.parametrize(
+            "answer, expect_raises",
+            [
+                (42, no_exception()),
+                (12, pytest.raises(WrongAnswer)),
+            ]
+        )
+        def test_search_for_the_meaning_of_life_universe_and_everything(
+            answer: int,
+            expect_raises: typing.ContextManager,
+        ):
             with expect_raises:
                 computer.validate_result(answer)
     """