diff --git a/src/Runner.Worker/ActionManager.cs b/src/Runner.Worker/ActionManager.cs
index 954d9a691f6..88af0347a74 100644
--- a/src/Runner.Worker/ActionManager.cs
+++ b/src/Runner.Worker/ActionManager.cs
@@ -1004,7 +1004,7 @@ private ActionSetupInfo PrepareRepositoryActionAsync(IExecutionContext execution
                 if (actionDefinitionData.Execution.ExecutionType == ActionExecutionType.Container)
                 {
                     var containerAction = actionDefinitionData.Execution as ContainerActionExecutionData;
-                    if (containerAction.Image.EndsWith("Dockerfile") || containerAction.Image.EndsWith("dockerfile"))
+                    if (DockerUtil.IsDockerfile(containerAction.Image))
                     {
                         var dockerFileFullPath = Path.Combine(actionEntryDirectory, containerAction.Image);
                         executionContext.Debug($"Dockerfile for action: '{dockerFileFullPath}'.");
diff --git a/src/Runner.Worker/Container/DockerUtil.cs b/src/Runner.Worker/Container/DockerUtil.cs
index 02c2ece5b79..af7de18df04 100644
--- a/src/Runner.Worker/Container/DockerUtil.cs
+++ b/src/Runner.Worker/Container/DockerUtil.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Text.RegularExpressions;
 
 namespace GitHub.Runner.Worker.Container
@@ -17,7 +18,7 @@ public static List<PortMapping> ParseDockerPort(IList<string> portMappingLines)
             string pattern = $"^(?<{targetPort}>\\d+)/(?<{proto}>\\w+) -> (?<{host}>.+):(?<{hostPort}>\\d+)$";
 
             List<PortMapping> portMappings = new List<PortMapping>();
-            foreach(var line in portMappingLines)
+            foreach (var line in portMappingLines)
             {
                 Match m = Regex.Match(line, pattern, RegexOptions.None, TimeSpan.FromSeconds(1));
                 if (m.Success)
@@ -61,5 +62,15 @@ public static string ParseRegistryHostnameFromImageName(string name)
             }
             return "";
         }
+
+        public static bool IsDockerfile(string image)
+        {
+            if (image.StartsWith("docker://", StringComparison.OrdinalIgnoreCase))
+            {
+                return false;
+            }
+            var imageWithoutPath = image.Split('/').Last();
+            return imageWithoutPath.StartsWith("Dockerfile.") || imageWithoutPath.StartsWith("dockerfile.") || imageWithoutPath.EndsWith("Dockerfile") || imageWithoutPath.EndsWith("dockerfile");
+        }
     }
 }
diff --git a/src/Runner.Worker/Handlers/ContainerActionHandler.cs b/src/Runner.Worker/Handlers/ContainerActionHandler.cs
index 3eaacb5a94c..d6494f63f4c 100644
--- a/src/Runner.Worker/Handlers/ContainerActionHandler.cs
+++ b/src/Runner.Worker/Handlers/ContainerActionHandler.cs
@@ -44,9 +44,8 @@ public async Task RunAsync(ActionRunStage stage)
             {
                 Data.Image = Data.Image.Substring("docker://".Length);
             }
-            else if (Data.Image.EndsWith("Dockerfile") || Data.Image.EndsWith("dockerfile"))
+            else if (DockerUtil.IsDockerfile(Data.Image))
             {
-                // ensure docker file exist
                 var dockerFile = Path.Combine(ActionDirectory, Data.Image);
                 ArgUtil.File(dockerFile, nameof(Data.Image));
 
@@ -68,6 +67,10 @@ public async Task RunAsync(ActionRunStage stage)
 
                 Data.Image = imageName;
             }
+            else
+            {
+                throw new InvalidOperationException($"'{Data.Image}' should be either '[path]/Dockerfile' or 'docker://image[:tag]'.");
+            }
 
             string type = Action.Type == Pipelines.ActionSourceType.Repository ? "Dockerfile" : "DockerHub";
             // Set extra telemetry base on the current context.
diff --git a/src/Test/L0/Container/DockerUtilL0.cs b/src/Test/L0/Container/DockerUtilL0.cs
index c069255a85c..c31e9aead81 100644
--- a/src/Test/L0/Container/DockerUtilL0.cs
+++ b/src/Test/L0/Container/DockerUtilL0.cs
@@ -144,5 +144,48 @@ public void ParseRegistryHostnameFromImageName(string input, string expected)
             var actual = DockerUtil.ParseRegistryHostnameFromImageName(input);
             Assert.Equal(expected, actual);
         }
+
+        [Theory]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        [InlineData("dockerhub/repo", false)]
+        [InlineData("debian:latest", false)]
+        [InlineData("something/dockerfileimage", false)]
+        [InlineData("ghcr.io/docker/dockerfile", true)] // should be false but might break the current workflows
+        [InlineData("Dockerfile", true)]
+        [InlineData("Dockerfile.", true)]
+        [InlineData(".Dockerfile", true)]
+        [InlineData(".Dockerfile.", false)]
+        [InlineData("dockerfile", true)]
+        [InlineData("dockerfile.", true)]
+        [InlineData(".dockerfile", true)]
+        [InlineData(".dockerfile.", false)]
+        [InlineData("Dockerfile.test", true)]
+        [InlineData("test.Dockerfile", true)]
+        [InlineData("docker/dockerfile:latest", false)]
+        [InlineData("/some/path/dockerfile:latest", false)]
+        [InlineData("dockerfile:latest", false)]
+        [InlineData("Dockerfile:latest", false)]
+        [InlineData("dockerfile-latest", false)]
+        [InlineData("Dockerfile-latest", false)]
+        [InlineData("dockerfile.latest", true)]
+        [InlineData("Dockerfile.latest", true)]
+        [InlineData("../dockerfile/dockerfileone", false)]
+        [InlineData("../Dockerfile/dockerfileone", false)]
+        [InlineData("../dockerfile.test", true)]
+        [InlineData("../Dockerfile.test", true)]
+        [InlineData("./dockerfile/image", false)]
+        [InlineData("./Dockerfile/image", false)]
+        [InlineData("example/Dockerfile.test", true)]
+        [InlineData("./example/Dockerfile.test", true)]
+        [InlineData("example/test.dockerfile", true)]
+        [InlineData("./example/test.dockerfile", true)]
+        [InlineData("docker://Dockerfile", false)]
+        [InlineData("docker://ubuntu:latest", false)]
+        public void IsDockerfile(string input, bool expected)
+        {
+            var actual = DockerUtil.IsDockerfile(input);
+            Assert.Equal(expected, actual);
+        }
     }
 }