Use a numeric USER instruction in Dockerfiles

[srguglielmo]

NOTE: This is a replacement for PR #1073 on master. As requested, only Dockerfile.txt is changed (not Dockerfile). Original PR text folllows:

Description

By using a numeric USER instruction in Dockerfiles, the images are now able to pass a strict Kubernetes securityContext configuration. Specifically, runAsNonRoot, which requires a numeric user/group ID. It is also a Docker best practice. The values of 1200 and 1201 are arbitrary.

Motivation and Context

While running a Selenium grid cluster in Kubernetes, I noticed I was unable to run the images with runAsNonRoot: true due to the following error:

CreateContainerConfigError: container has runAsNonRoot and image has non-numeric user (seluser), cannot verify user is non-root

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

Here's an example of the k8s config that runs successfully after this patch:

apiVersion: apps/v1
kind: Deployment
spec:
  template:
    spec:
      containers:
      - name: hub
        image: selenium/hub:local
        securityContext:
          allowPrivilegeEscalation: false
          capabilities:
            drop:
            - ALL
          privileged: false
          readOnlyRootFilesystem: false
          runAsNonRoot: true

I can run VERSION=local make build locally successfully. Let me know if there's anything I can change or if this can be tested somehow.

Thanks,
Steve

[diemol]

Thank you @srguglielmo!