Converting docker tar file into a singularity image?

Question

I am new to both Docker and Singularity. I recently created the canonical main.c.

#include <stdio.h>
#include <stdlib.h>

int main(void){
    printf("Hello Docker World!\n");
    return 0;
}

I statically compiled this code, i.e.

gcc -static -static-libgcc -static-libstdc++ -o hello main.c

I then built the docker image and could run it, i.e.

dockerd &  ## Start Docker daemon
docker build --tag hello .
docker run hello   ## Outputs "Hello Docker World"

I then save the image so that I can export it to a second computer (which does not have docker, but does have singularity), i.e.

docker save hello > hello.tar

Now on the second machine, which does not have docker but does have singularity, I want to create a singularity image. Tthe singularity documentation give instructions on creating a singularity image from a docker image on Docker Hub, but they do not give instructions on converting from a docker tar'd file.

Question : How would I create a singularity image from my hello.tar

Answer 1

1. First, save your docker image.

   sudo docker save image_id -o local.tar

2. Then copy to another machine in any way you like

3. Finally, build a singularity image from the local.tar by

   singularity build local_tar.sif docker-archive://local.tar

   which used the docker-archive bootstrap agent. Can read it here