I don't know that much about the state of container systems these days, but to me it seems it's the "comfortable" way in the "security vs comfort" tradeoff. Use where applicable and hope for better days.
I don't think you should put security at any case in tradeoff, by your argument one can save passwords in plaintext because its easy and comfortable, but its just not acceptable and this would effect not only your app but every app that your users use.
Think about your server consuming resources: It needs a [machine] password stored someplace in order to authenticate itself to those resources. If it stores this encrypted, then an operator must be present to decrypt it when (re)starting the server. If this is stored unencrypted, then it is not and the server can be started unattended (or autoscaled or whatever). That is a security tradeoff because there are both real benefits and risks gained and lost with each approach.
If docker was ever a tradeoff, it was between taking the time to have a thoughtful architecture versus trying to get acquired, and I think we know which way they went.