docker: buildworker: add missing file package

docker: buildworker: add missing file package

In Debian 11 (Bullseye), the `file` package was installed indirectly
because the `man-db` package recommended it. The `man-db` package is
often installed as a dependency of other packages, such as
`build-essential` and tools like `gcc` and `make`.  By default,
`apt-get` installs recommended packages, so `file` was installed
automatically.

Starting from Debian 12 (Bookworm), the `man-db` package no longer
recommends the `file` package. This change was made to reduce the size
of minimal container images and installations where `file` is not
essential.

So lets install this package explicitly.

Signed-off-by: Petr Štetiar <ynezz@true.cz>