A backport of Git for Jessie is possible:
15:58:49 <Dereckson> checkbackport git --verbose
15:58:50 <judd> Backporting package git in sid→jessie/amd64: all build-dependencies satisfied using jessie, virtual.
15:58:51 <judd> jessie: libpcre3-dev,gettext,libcurl4-gnutls-dev,libexpat1-dev,subversion,libsvn-perl,libyaml-perl,tcl,libhttp-date-perl | libtime-modules-perl,python,cvs,cvsps,libdbd-sqlite3-perl,unzip,libio-pty-perl,dpkg-dev (>= 1.16.2~),asciidoc,xmlto,docbook-xsl; virtual: libz-dev
So, I'd recommed to follow https://blog.codeship.com/using-docker-build-debian-packages/ and maintain a Debian packages repository for such purpose.
Specific instructions to build the Git package: http://superuser.com/a/644618/250330
Currently, the image runs on Buster, so we're more up to date:
$ git --version
git version 2.20.1
Backport version is 2.27 while I see 2.28 on Cygwin, FreeBSD, 2.25 on Eglide if we want to see what will happen with a non updated Sid image container.
The annoying thing isn't we'd have a one version lag, but the delta between release date and backport available: 2.28 has been released Mon, 27 Jul 2020 09:49:16 -0700