packaging: drop Disco (19.04) and add Focal (20.04)
Disco support ended in January 2020, and Focal does not have an announced EOL.
Something is now installing and configuring `tzdata`, which was throwing up an
interactive prompt to configure the timezone. Aside from being hostile to
automation, the prompt didn't actually accept input and hung the process. This
propagates the host's timezone into the image via environment variable in order
to skip the prompt, and avoid hardcoding a value.
Differential Revision: https://phab.mercurial-scm.org/D9396
{
// Enforcing
"eqeqeq" : true, // true: Require triple equals (===) for comparison
"forin" : true, // true: Require filtering for..in loops with obj.hasOwnProperty()
"freeze" : true, // true: prohibits overwriting prototypes of native objects such as Array, Date etc.
"nonbsp" : true, // true: Prohibit "non-breaking whitespace" characters.
"undef" : true, // true: Require all non-global variables to be declared (prevents global leaks)
// Environments
"browser" : true // Web Browser (window, document, etc)
}