test: validate that the timezone in the base image is UTC

[jamietanna]

We should protect ourselves from any potential breakage in the future by ensuring that we are building an image with the timezone set to UTC.

If a user wants to override that in the future they can, but either containerbase/ubuntu or containerbase/base should enforce this.

(We currently do build a UTC image, this would protect against regressions)

[viceice]

how did we test this 🤔

[jamietanna]

I assume we can do something like check the value of /etc/localtime:

% docker run   -ti ghcr.io/renovatebot/renovate:43.158.2 bash
ubuntu@7b746ec1a074:/usr/src/app$ ls -al /etc/localtime
lrwxrwxrwx 1 root root 27 Apr 24 14:47 /etc/localtime -> /usr/share/zoneinfo/Etc/UTC

?