Small Docker container

A note on a small Docker image container

Contents of scratch

I tried the following to make sure the contents of scratch were completely empty.

$ uname -sm
Darwin x86_64
$ GOOS=linux GOARCH=arm64 go build -o hello hello.go
$ cat > Dockerfile
FROM scratch

COPY hello /hello
ENTRYPOINT ["/hello"]
$ docker image build -t test/hello .

$ docker image ls
test/hello latest b5590fa74374 2 hours ago 2.04MB

$ docker run test/hello
Hello, World

$ docker image save -o out.tar test/hello
$ ls -lhn out.tar
-rw-------  1 501  20   2.0M  9 10 18:12 out.tar

$ tar xvf out.tar 
x 53d2a1a74edd0b6d3863682083df76fa417ebe730feecd84a55309fe407f7b0d/
x 53d2a1a74edd0b6d3863682083df76fa417ebe730feecd84a55309fe407f7b0d/VERSION
x 53d2a1a74edd0b6d3863682083df76fa417ebe730feecd84a55309fe407f7b0d/json
x 53d2a1a74edd0b6d3863682083df76fa417ebe730feecd84a55309fe407f7b0d/layer.tar
x b5590fa74374835c813ed2c8490b2997581d3b815d306be739a2be67f6664792.json
x manifest.json
x repositories

$ tar tvf 53d2a1a74edd0b6d3863682083df76fa417ebe730feecd84a55309fe407f7b0d/layer.tar
-rwxr-xr-x  0 0      0     2038890  9 10 16:28 hello

Make a small container from a tar ball

Since it is a container, it will work if you import the chroot environment into tar.

$ mkdir chroot
$ GOOS=linux GOARCH=arm64 go build -o chroot/hello hello.go
$ ls -lRhn chroot
total 4104
-rwxr-xr-x  1 501  20   2.0M  9 10 18:21 hello

$ (cd chroot; tar cf ../image.tar .)
$ ls -lhn image.tar 
-rw-r--r--  1 501  20   2.0M  9 10 18:24 image.tar

$ docker image import image.tar test/import

$ docker image ls test/import
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
test/import         latest              bb6be6e60bb1        19 seconds ago      2.1MB

$ docker run test/import /hello
Hello, World

Worked well

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