gnupg on read-only filesystem

[Fourhundred Thecat]

Hello,

I am using gnupg 2.1.18-6 on Debian Stretch.

My root partition (/) is mounted read-only and I cannot use gpg as root,
because gpg wants to start gpg-agent and write to /root/.gnupg/

ie:

  gpg -d file.gpg

  gpg: error creating keybox '/root/.gnupg/pubring.kbx': Read-only file
system
  gpg: keyblock resource '/root/.gnupg/pubring.kbx': Read-only file system
  gpg: can't connect to the agent: IPC connect call failed
  gpg: problem with the agent: No agent running
  gpg: decryption failed: No secret key

With gpg version 1, I could use --lock-never and --no-use-agent and it
worked on read-only filesystem.

How can I use gpg version 2 on read-only filesystem and without agent ?

thanks,