add apt update so Julia gets found and installed by Docker

[valeriupredoi]

Description

Automatic builds are failing (see issue) so Bouwe alerted me and coopted me onto the Docker side, his idea works too - a simple apt-get update is enough to fix the Julia pkg installation (works locally on me laptop).

• Closes Automatic builds on DockerHub are failing #2276 - I have not finished the docker container build just yet (locally) but the Julia bit is fixed and the package gets installed before the esmvaltool image build

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[bouweandela]

Thanks!

[zklaus]

Note that there now also is an up-to-date conda-forge package for julia, though most library packages are still missing.

[valeriupredoi]

my env was still solving in the container yesterday when I left home, and now I am stuck home coz Royal Mail is late delivering a parcel - anyways - off topic, I merged since there were no changes in the actual conda env building in the container, so it might be that my laptop is being silly, or conda is silly, as usual 😁 Thanks @zklaus - we'll have to change the Julia install to conda soon, but I gather that the Julia conda package is not yet fully complete with her dependencies?

[zklaus]

Right. I didn't mean to cause any change right now. Just keepin' you on the up'n'up.

[valeriupredoi]

cheers @zklaus 🍺 OK now we need to push the new 2.3.1 docker container, Klaus you want to do the honors or should I do it?
EDIT: nevermind, the auto build failed, gotta first understand why, conda hitting back again 🤦‍♂️