gpg signing fails when trying to publish a snapshot

[Joost Ringoot]

Following this tutorial
https://www.aptly.info/tutorial/mirror/

I created a new key but was unable to publish a mirror snapshot, because aptly doesn't find the default secret key

root@len-003:~# apt list aptly

Listing... Done

aptly/stable,stable,now 1.3.0+ds1-2.2~deb10u1 amd64 [installed]

aptly/stable,stable 1.3.0+ds1-2.2~deb10u1 i386

root@len-003:~# 

root@len-003:~# aptly snapshot create eid-buster-amd64-snap from mirror eid-buster-amd64

Snapshot eid-buster-amd64-snap successfully created.

You can run 'aptly publish snapshot eid-buster-amd64-snap' to publish snapshot as Debian repository.

root@len-003:~# aptly publish snapshot eid-buster-amd64-snap

Loading packages...

Generating metadata files and linking package files...

Finalizing metadata files...

Signing file 'Release' with gpg, please enter your passphrase when prompted:

gpg: no default secret key: secret key not available

gpg: signing failed: secret key not available

ERROR: unable to publish: unable to detached sign file: exit status 2

root@len-003:~# gpg --list-keys

gpg: checking the trustdb

gpg: marginals needed: 3  completes needed: 1  trust model: pgp

gpg: depth: 0  valid:   1  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 1u

/root/.gnupg/pubring.kbx

------------------------

pub   rsa3072 2020-02-07 [SC]

      62034D0EC6B823A3630662CA72360C179712C1A1

uid           [ultimate] Joost Ringoot (RMI aptly snapshot signer) <joost.ringoot_at_meteo.be>

sub   rsa3072 2020-02-07 [E]

root@len-003:~# 

.

trying it again gives a shorter error message:

root@len-003:~# aptly publish snapshot eid-buster-amd64-snap

ERROR: unable to initialize GPG signer: looks like there are no keys in gpg, please create one (official manual: http://www.gnupg.org/gph/en/manual.html)

but there is a key that can sign

root@len-003:~# gpg --sign test.txt

root@len-003:~# gpg --verify test.txt.gpg

gpg: Signature made Fri 07 Feb 2020 11:45:42 CET

gpg:                using RSA key 62034D0EC6B823A3630662CA72360C179712C1A1

gpg: Good signature from "Joost Ringoot (RMI aptly snapshot signer) <joost.ringoot_at_meteo.be>" [ultimate]

root@len-003:~# 

Even explicitly defining the keyring does not appear to work:

root@len-003:~# aptly publish snapshot eid-buster-amd64-snap -secret-keyring="/root/.gnupg/pubring.kbx"

ERROR: unable to initialize GPG signer: looks like there are no keys in gpg, please create one (official manual: http://www.gnupg.org/gph/en/manual.html)

root@len-003:~# 

Questions: 
- suggestions to make aptly use that key?

- How to make a key that aptly would use?
- alternatively: how does aptly call gpg in the source code? (couldn't find it), how does it select the keyring and the key?


Thanks,

Joost