Fix Docker image security issue

Right now, in the Dockerfile the user that executes the command is root instead of node. This may pose a security issue and needs to be fixed.

The reason behind this is related to the saving of media in the file system when useing Docker compose. In this scenario, the folder in which files are saved is mounted as volume. Docker compose exclusively mounts named volumes as root (as detailed here). For this reason, whenever the Bot running as node user tryes to save a file in tthe folder, it receives a permission error.