Bump Ubuntu/Python versions in ECR GC Docker image (pytorch#58309)

Summary:
Should fix the ECR GC jobs that broke as a result of pytorch#58275; examples:

- https://app.circleci.com/pipelines/github/pytorch/pytorch/322385/workflows/c26788cb-2147-4279-9813-224af3c01583/jobs/13480923
- https://app.circleci.com/pipelines/github/pytorch/pytorch/322385/workflows/c26788cb-2147-4279-9813-224af3c01583/jobs/13473074
- https://app.circleci.com/pipelines/github/pytorch/pytorch/322385/workflows/c26788cb-2147-4279-9813-224af3c01583/jobs/13473077
- https://app.circleci.com/pipelines/github/pytorch/pytorch/322385/workflows/c26788cb-2147-4279-9813-224af3c01583/jobs/13473073

See also pytorch#58308.

Pull Request resolved: pytorch#58309

Reviewed By: malfet, seemethere

Differential Revision: D28447014

Pulled By: samestep

fbshipit-source-id: db857154a94482f4da1db8d74c383527d1b14b49

.circleci/ecr_gc_docker/Dockerfile

@@ -1,6 +1,6 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 
-RUN apt-get update && apt-get install -y python-pip git && rm -rf /var/lib/apt/lists/* /var/log/dpkg.log
+RUN apt-get update && apt-get install -y python3-pip git && rm -rf /var/lib/apt/lists/* /var/log/dpkg.log
 
 ADD requirements.txt /requirements.txt