Support using kubernetes volume mounts for configuration settings

[sboardwell]

Would there be interest in discussing using kubernetes volume mounts for configuration in addition to environment variables and flags? Please tell if this is too far fetched but...

• although envFrom: {} exists, I do not generally like to have credentials as environment variables
• using the --password-file allows pointing to a file
  • in kubernetes, this would involved mounting a secret
• if we are mounting anyway, why not have:
  • (less work) a single flag --config-file pointing to a file containing the environment variables GIT_SYNC_USERNAME=my-user á la https://kubernetes.io/docs/tasks/configmap-secret/managing-secret-using-config-file/#specify-unencoded-data-when-creating-a-secret
  • (more work) point to a directory --config-dir where all the configuration values are files /secret/mount/GIT_SYNC_USERNAME containing the username, etc (kubernetes allows key mapping if necessary with https://kubernetes.io/docs/tasks/inject-data-application/distribute-credentials-secure/#project-secret-keys-to-specific-file-paths)

The advantages I see are:

• the values can be kept in a single place (separation of concerns - config vs app)
• the config file could be a secret, meaning it would be "slightly" more secure - one could choose which bits are shown as env vars, and which are read
• mounts are updated automatically, potentially opening the door on-demand changes without restarting the pod
  • https://kubernetes.io/docs/concepts/configuration/configmap/#mounted-configmaps-are-updated-automatically
  • imagine being able to change the log level temporarily on a live instance
  • this would need some config reload logic in the app, but still an exciting prospect IMO

Originally posted by @sboardwell in #752 (comment)

[thockin]

I don't object to this but I have some thoughts (shocking, I know):

• most of the config is not "secret", so does that imply 2 different configs?
• do we need a generic "KEY=value" mechanism or is it actually a schema?
• If the config file changes, do we feel obligated to restart? Most of those configs can't be live-updated (without massive changes to the app)

[sboardwell]

Hi, sorry for the radio silence. A few things on at the moment. Will definitely get onto this and #752 in the next few days.

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