Hi @Ben_Luk, and welcome to the forum!
You can install mongocryptd from apt-get on Ubuntu. For example in a Dockerfile:
RUN wget -qO - https://www.mongodb.org/static/pgp/server-4.4.asc | apt-key add -
RUN echo "deb [ arch=amd64,arm64,s390x ] http://repo.mongodb.com/apt/ubuntu focal/mongodb-enterprise/4.4 multiverse" | tee /etc/apt/sources.list.d/mongodb-enterprise.list
RUN apt-get update && apt-get install -y mongodb-enterprise-cryptd=4.4.1
See also github.com/sindbach/field-level-encryption-docker/node for a working example.
Regards,
Wan