This way, when the language is written form right-to-left, elements
using Foundation mixins/classes will float to the opposite direction as
they do in left-to-right languages. The same will apply to text
alignment.

To offer full support for RTL languages, we need to change every single
reference to `float: left`, `float: right`, `text-align: left`,
`text-align: right`, and possible adjust other properties like `left`,
`margin-left`, `padding-left` or `border-left`. In the meantime, we at
least partially support these languages.

Replacing `float` with `flex` when possible would also improve RTL
support.

This way properties flex layouts text direction will work properly.

Looks like our test suite wasn't executed in commit 4dbc027, and so we
weren't notified of these failures.

Debian Bullseye was released two days ago, and is now the default
distribution for the Docker image.

Our image isn't compatible with Debian Bullseye right now, and we
haven't done any testing with it, so for now we're staying with Buster.

Fix tests after replacing message with notification

Explicitely use Debian Buster in Dockerfile

Improve support for RTL languages

Note this version includes Bundler 2, so we can finally upgrade.

We were getting hundreds of "warning: URI.escape is obsolete" messages.
So we're using `URI::DEFAULT_PARSER.escape` instead.

IMHO it's OK to add this monkey-patch because we're replacing Paperclip
with Active Storage, and when we finish with that we'll delete this
file.