E: gnupg, gnupg2 and gnupg1 do not seem to be installed, but one of them is required for this operation

In your Dockerfile, run this first:

apt-get update && apt-get install -y gnupg2

I faced the same issue:

E: gnupg, gnupg2 and gnupg1 do not seem to be installed, but one of them is required for this operation

I resolved by using the following commands:

apt-get update
apt-get install gnupg

In addition to existing answers:

RUN apt-get update && apt-get install -y gnupg

-y flag agrees to terms during installation process. It is important not to break the build