Hey @Haazeel, welcome to the community!
Full disclosure, I have not done this myself yet so I checked with @harshil1712 on this one 
Additional packages would need to be installed in the same place n8n is installed to which is /usr/local/lib/node_modules/n8n. So you should be able to add it to your custom docker image using a command like RUN cd /usr/local/lib/node_modules/n8n && npm install @digital-boss/n8n-nodes-zammad (instead of the current npm install -g).