## Problem
In several workflows, we have repeating code which is separated into
two steps:
```bash
mkdir -p $(pwd)/.docker-custom
echo DOCKER_CONFIG=/tmp/.docker-custom >> $GITHUB_ENV
...
rm -rf $(pwd)/.docker-custom
```
Such copy-paste is prone to errors; for example, in one case, instead of
`$(pwd)/.docker-custom`, we use `/tmp/.docker-custom`, which is shared
between workflows.
## Summary of changes
- Create a new action `actions/set-docker-config-dir`, which sets
`DOCKER_CONFIG` and deletes it in a Post action part
