Do not use non-numeric UID in Dockerfile

[jiridanek]

https://docs.openshift.com/container-platform/4.6/openshift_images/create-images.html says

Support arbitrary user ids

By default, OpenShift Container Platform runs containers using an arbitrarily assigned user ID. This provides additional security against processes escaping the container due to a container engine vulnerability and thereby achieving escalated permissions on the host node.

For an image to support running as an arbitrary user, directories and files that are written to by processes in the image must be owned by the root group and be read/writable by that group. Files to be executed must also have group execute permissions.

Adding the following to your Dockerfile sets the directory and file permissions to allow users in the root group to access them in the built image:

RUN chgrp -R 0 /some/directory &&
chmod -R g=u /some/directory
Because the container user is always a member of the root group, the container user can read and write these files.

[jiridanek]

This is to fix rh-messaging/shipshape#75

[jiridanek]

Actually, correct fix is

    shadow-utils \
 && microdnf clean all

RUN useradd --uid 10000 runner
USER 10000

as in https://github.com/skupperproject/skupper-router/pull/827/files

This follows the least-privilege in any Kubernetes, not just OpenShift. The idea with 10000 is not to clash with any user defined on the host system, https://kubesec.io/basics/containers-securitycontext-runasuser/