To install this plugin you need RouterOS 7.
Before installing update RouterOS to the latest minor update of version 7.

log in to the interface (web or winbox) go to System:Packages.
press Check_For_Updates, press Download

When you now go back to System:Packages you will see the non installed packages as grayed-out wheras before you could only see the installed packages. You can now select and install the package container

Create the tmp directory by placing a file there:

/file
add name="usb1/tmp" contents="starter file"

setup the container registry to use, and the scratch dir

/container/config
set registry-url=https://registry-1.docker.io tmpdir=usb1/tmp

To activate the container plugin you need to enable it:

/system/device-mode/update container=yes

this ends with the intruction to press the mode button, however this does not seem to work, so go over to the router and pull the power cord and replace it after 10 seconds.

after the router has booted back up check if the container plugin is activated:

/system/device-mode/print

the output should indicate yes at the bottom :

[abel@styx] > /system/device-mode/print
mode: advanced
allowed-versions: 7.13+,6.49.8+
...
container: ye

https://help.mikrotik.com/docs/spaces/ROS/pages/40992872/Packages#Packages-Downloaddirectlyfromtherouter