Update Linux desktop Dockerfile for GTK switch

[stuartmorgan-g]

Description

Once the switch to GTK lands, libgtk-3-dev will be needed for building.

Related Issues

Part of flutter/flutter#54860

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[robert-ancell]

I just read the apt man page and noticed it says:

All features of apt(8) are available in dedicated APT tools like apt-get(8) and apt-cache(8) as well. apt(8) just changes the default value of some options (see apt.conf(5) and specifically the Binary scope). So you should prefer using these commands (potentially with some additional options enabled) in your scripts as they keep backward compatibility as much as possible.

So I was wrong when I said you should switch to apt, it turns out apt-get is the correct command to use.

Other than that LGTM.

[robert-ancell]

LGTM