watchtower/docs/lifecycle-hooks.md

Executing commands before and after updating

DO NOTE: Both commands are shell commands executed with sh, and therefore require the container to provide the sh executable.

It is possible to execute a pre-update command and a post-update command inside every container updated by watchtower. The pre-update command is executed before stopping the container, and the post-update command is executed after restarting the container.

This feature is disabled by default. To enable it, you need to set the option --enable-lifecycle-hooks on the command line, or set the environment variable WATCHTOWER_LIFECYCLE_HOOKS to true.

Specifying update commands

The commands are specified using docker container labels, with com.centurylinklabs.watchtower.lifecycle.pre-update-command for the pre-update command and com.centurylinklabs.watchtower.lifecycle.post-update for the post-update command.

These labels can be declared as instructions in a Dockerfile:

LABEL com.centurylinklabs.watchtower.lifecycle.pre-update="/dump-data.sh"
LABEL com.centurylinklabs.watchtower.lifecycle.post-update="/restore-data.sh"

Or be specified as part of the docker run command line:

docker run -d \
  --label=com.centurylinklabs.watchtower.lifecycle.pre-update="/dump-data.sh" \
  --label=com.centurylinklabs.watchtower.lifecycle.post-update="/restore-data.sh" \
  someimage

Execution failure

The failure of a command to execute, identified by an exit code different than 0, will not prevent watchtower from updating the container. Only an error log statement containing the exit code will be reported.