the canonical documentation for secrets is at docs.drone.io. Blogs posts, discourse posts, etc are point-in-time and could be outdated. This is (at the time of responding) the proper syntax for vault secrets:
---
kind: secret
name: username
get:
path: secret/data/docker
name: username
---
kind: secret
name: password
get:
path: secret/data/docker
name: password
one thing that jumps out is that ${variable} is subject to interpolation and needs to be escaped, similar to docker-compose, like this:
-commands: - echo "${MY_SSH_KEY}"
+commands: - echo "$${MY_SSH_KEY}"