container entrypont should add `-config` when using agent

[briantist]

Is your feature request related to a problem? Please describe. When using the vault container to run agent, along with VAULT_LOCAL_CONFIG, the entrypoint does write the config file to the default location, but unlike the server subcommand, using the agent subcommand does not automatically add the -config argument telling Vault where to find it.

Describe the solution you'd like I would like to be able to use docker run -e VAULT_LOCAL_CONFIG="$MY_CONFIG" vault vault agent where $MY_CONFIG contains the agent config.

Describe alternatives you've considered I tried to set the command to variations of vault agent -config="$VAULT_CONFIG_DIR/local.json" but there are two problems with that:

1. The variable $VAULT_CONFIG_DIR is set within the entrypoint but when invoke this way it does not seem to be able to expand it.
2. I still have to hardcode the name of the generated config file which could change.

In the end, I have to invoke the command as vault agent -config=/vault/config/local.json which hardcodes everything.

Explain any additional use-cases If there are any use-cases that would help us understand the use/need/value please share them as they can help us decide on acceptance and prioritization.

Additional context Add any other context or screenshots about the feature request here.