If you want o debug every step of image creation using packer, you can use the
packer build -debug app.json
This debug mode will enable you to select actions on you own.
One thing about debug mode is that parallelization is disabled for debug mode.
Also you can setup logging for your image builds. Set the following parameters from the terminal.
export PACKER_LOG_PATH="/path/to/packer.log" export PACKER_LOG=10