locale of .utf8 or .UTF-8?

Question

Of the same Debian 11 bullseye, I get different locale for C: .utf8 and .UTF-8:

My machine:

$ locale -a
C
C.utf8
POSIX
en_US
en_US.iso88591
en_US.utf8
$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 11 (bullseye)
Release:        11
Codename:       bullseye

Yet, in its docker container, I got:

$ locale -a
C
C.UTF-8
POSIX
en_US
en_US.iso88591
en_US.utf8
$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 11 (bullseye)
Release:        11
Codename:       bullseye

I'm wondering where the difference is coming from and how to get them the same.

My bullseye docker container is only built 2 weeks ago:

$ docker images
REPOSITORY     TAG       IMAGE ID       CREATED       SIZE
. . .
debian         latest    54e726b437fb   2 weeks ago   124MB