Declared volumes make deploying image as "pre-built" db extremely difficult

[mshafer-NI]

High level

I'm trying to build a db into an image to deploy (read-only) in a k8s cluster. I need to be able to build it one environment and deploy in a completely different one.

Desired behavior

Ability to (within my Dockerfile):

• start FROM mongo,
• launch the db in background (can use tmux for this)
• run script to populate db
• stop db
• commit as image

Blockers currently (/ other solutions if they would work)

• Because these images declare volumes for /data/db,
  • Docker generates an ephemeral volume whenever a descendant image is launched.
  • Docker appears to not allow a client Dockerfile to "reset" volumes (like we can entrypoint and cmd),
• Nor have I been able to figure out how to get mongod to store the initial db in a different folder.

[tianon]

The easiest workaround is to pass --dbpath to a different path, which all our scripts (and mongod itself) will honor:

mongo/docker-entrypoint.sh

Lines 224 to 228 in 2a7ebf4

	if ! dbPath="$(_mongod_hack_get_arg_val --dbpath "$@")"; then
	if _parse_config "$@"; then
	dbPath="$(jq -r '.storage.dbPath // empty' "$jsonConfigFile")"
	fi
	fi

(Otherwise, this is a duplicate of #306)