Timezone conversion on Ubuntu 20.04

Hi all,

I am testing R 4.0 and ran into an issue with timezones on Ubuntu
Focal: converting a timestamp to another timezone results in NA:

  as.POSIXct(as.POSIXlt(Sys.time(), tz = "CET"), tz = "EST")

This only happens on Ubuntu Focal, it seems to work fine on Ubuntu
Bionic. I am the standard ubuntu docker image icw/ r-base from Dirk's
ppa:edd/r-4.0 on both systems.

Am I missing a system dependency for tzone data?

Jeroen