Packer fails my docker build with error "sudo: not found" despite sudo being present - docker

Question

I'm trying to build a packer image with docker on it and I want docker to create a docker image with a custom script. The relevant portion of my code is (note that the top builder double-checks that sudo is installed):
{
"type": "shell",
"inline": [
"apt-get install sudo"
]
},
{
"type": "docker",
"image": "python:3",
"commit": true,
"changes": [
"RUN pip install Flask",
"CMD [\"python\", \"echo.py\"]"
]
}
The relevant portion of my screen output is:
==> docker: provisioning with shell script: /var/folders/s8/g1_gobbldygook/T/packer-shell23453453245
docker: /temp/script_1234.sh: 3: /tmp/script_1234.sh: sudo: not found
==> docker: killing the contaner: 34234hashvomit234234
Build 'docker' errored: Scipt exited with non-zero exit status: 127
The script in question is not one of mine. It's some randomly generated script that has a different series of four numbers every time I build. I'm new to both packer and docker, so maybe it's obvious what the problem is, but it's not to me.

Answer 1

There seem to be a few problems with your packer input. Since you haven't included the complete input file it's hard to tell, but notice a couple of things:
You probably need to run apt-get update before calling apt-get install sudo. Without that, even if the image has cached package metadata it is probably stale. If I try to build an image using your input it fails with:
E: Unable to locate package sudo
While not a problem in this context, it's good to explicitly include -y on the apt-get command line when you're running it non-interactively:
apt-get -y install sudo
In situations where apt-get is attached to a terminal, this will prevent it from prompting for confirmation. This is not a necessary change to your input, but I figure it's good to be explicit.
Based on the docs and on my testing, you can't include a RUN statement in the changes block of a docker builder. That fails with:
Stderr: Error response from daemon: run is not a valid change command
Fortunately, we can move that pip install command into a shell provisioner.
With those changes, the following input successfully builds an image:
{
"builders": [{
"type": "docker",
"image": "python:3",
"commit": true
}],
"provisioners": [{
"type": "shell",
"inline": [
"apt-get update",
"apt-get -y install sudo",
"pip install Flask"
]
}],
"post-processors": [[ {
"type": "docker-tag",
"repository": "packer-test",
"tag": "latest"
} ]]
}
(NB: Tested using Packer v1.3.5)