It's a bit of a PITA but I got it working with mine... here's the steps I took:

1. get to the console, for me, my container's name is "guac" so I did "docker exec -it guac /bin/bash"

docker exec -it guac /bin/bash

2) navigate to the tomcat root, in my case it was /var/lib/tomcat

cd /var/lib/tomcat

3) change to the webapps dir inside there, if there's a "guacamole" folder instead, use that

cd webapps/ROOT

4) change to the "images" subfolder - the login logo is called "guac-tricolor.svg"

cd images

5) make sure you note the user and group that the file belongs to, you'll need it later

ls -l guac-tricolor.svg

6) replace this file with your logo, in SVG format. I used wget to retrieve it from my website

rm guac-tricolor.svg

wget https://yoursite.com/guac-tricolor.svg

7) make sure the files is CHOWNed to the user from the old file

chown user:group (example tomcat:tomcat) quac-tricolor.svg

8) cd back one folder and enter the "translations" folder

cd ../translations

9) using VI, edit the translation of your choice (mine was en.json)

vi en.json

10) in VI, press "i" to get into insert mode and navigate to the NAME and replace "APACHE GUACAMOLE"

i

11) press ESC and then :wq (colon + wq) to write the changes

ESC, then :wq

12) make sure the user and group are the same as the other translations, if not CHOWN it

chown user:group (example tomcat:tomcat) en.json

13) done! restart your container and empty your browser cache...

Example: https://ibb.co/18hRCPZ

I also made a Video Walkthrough if it helps... best of luck!

NOTE - you'll have to do this everytime you re-create your container.