For a long time, I’ve built an app using Docker with an intermediate container e.g. I’d build myapp:base which has all the core files copied:
FROM ubuntu:17.10
ADD app /app
Now, the main container myapp:release might be created simply with this
FROM myapp:base
ADD app /app
A handful of files might be modified in the app directory, and I’d wind up with a small extra layer.
Recently though, this final ADD has resulted in a much larger layer, and I suspect it’s related to a Docker update (I’m using Docker version 18.02.0-ce, build fc4de44, running on Container Linux 1702.1.0)
Has something changed in how Docker figures out whats different? How can I get back to an efficient build which just has a small number of updated files in a layer?
Steps to reproduce:
#make a dummy dir with a 64kb file in it
mkdir -p files
truncate -s 64k files/64k.file
#base container has a copy of the files
cat << EOF > Dockerfile.base
FROM ubuntu:16.04
COPY files/ /root/
EOF
#derived container should just have any updates
cat << EOF > Dockerfile.derived
FROM q899941:base
COPY files/ /root/
EOF
#build them....
docker build --file Dockerfile.base -t q899941:base .
docker build --file Dockerfile.derived -t q899941:derived .
#now let's review the layers
docker history q899941:derived
I get this result
IMAGE CREATED CREATED BY SIZE
4e1eb5168d55 Less than a second ago /bin/sh -c #(nop) COPY dir:8e20ede288278c71e… 65.5kB
022626ac5cf0 Less than a second ago /bin/sh -c #(nop) COPY dir:8e20ede288278c71e… 65.5kB
0458a4468cbc 5 weeks ago /bin/sh -c #(nop) CMD ["/bin/bash"] 0B
<missing> 5 weeks ago /bin/sh -c mkdir -p /run/systemd && echo 'do… 7B
<missing> 5 weeks ago /bin/sh -c sed -i 's/^#\s*\(deb.*universe\)$… 2.76kB
<missing> 5 weeks ago /bin/sh -c rm -rf /var/lib/apt/lists/* 0B
<missing> 5 weeks ago /bin/sh -c set -xe && echo '#!/bin/sh' > /… 745B
<missing> 5 weeks ago /bin/sh -c #(nop) ADD file:a3344b835ea6fdc56… 112MB
The top layer is 64kb even though no files changed.