Lambda Function built using Docker Image cannot find the lambda handler function

I am trying to test my Lambda function that was created using a Docker Image on a file that was uploaded to an S3 bucket. I double checked my CMD in the Dockerfile contents and the file/method name are both spelled correctly. Due to the need to install non-Python libraries for the Lambda function to run, a Docker Image rather than a zip file is required. I checked CloudWatch and the log events show the following error: INIT_REPORT Init Duration: 4.18 ms Phase: init Status: error Error Type: Runtime.InvalidEntrypoint, which suggests that the handler function isn't being located. I have tried providing the file_name.method_name (without file extensions) explicitly in the management console. No matter what permissions I grant, the handler function is unable to be located.