The scratch image is the most minimal image in Docker. This is the base ancestor for all other images. The scratch image is actually empty. It doesn’t contain any folders/files …
The scratch image is mostly used for building other base images. For instance, the debian image is built from scratch as such:
FROM scratch
ADD rootfs.tar.xz /
CMD ["bash"]
The rootfs.tar.xz contains all the files system files. The Debian image adds the filesystem folders to the scratch image, which is empty.
As I understand it is essentially blank. How is the binary executed in
the hello-world image then?
The scratch image is blank.The hello-world executable added to the scratch image is actually statically compiled, meaning that it is self-contained and doesn’t need any additional libraries to execute.
As stated in the offical docker docs:
Assuming you built the “hello” executable example from the Docker
GitHub example C-source code, and you compiled it with the -static
flag, you can then build this Docker image using: docker build –tag
hello
This confirms that the hello-world executable is statically compiled. For more info about static compiling, read here.