Change USER to use the numeric user in container image

[superbrothers]

What this PR does / why we need it:

This PR changes USER to use the numeric user in container image.

Currently USER is specified as nobody (65534). Therefore, this container image runs as a non-root user. However, Pod SecurityContext's RunAsNonRoot policy does not consider to be "run as non-root" if USER is specified as non-numric.

Error: container has runAsNonRoot and image has non-numeric user (nobody), cannot verify user is non-root

The UID of nobody is 65534.

$ docker run -it --rm --entrypoint=id caicloud/event-exporter:v1.0.0
uid=65534(nobody) gid=65534(nogroup) groups=65534(nogroup)

Which issue(s) this PR is related to (optional, link to 3rd issue(s)):

Fixes #

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Release note:

NONE