How to 'squash' base image layers into a single layer? - docker

Question

I am trying to upgrade the base image version in one of my images, but due to a layer limit of 40, my builds are failing.
I noticed that in the older image, docker history gave this output:
IMAGE CREATED CREATED BY SIZE
092c6c14cf83 8 days ago /bin/sh -c #(nop) ENV LANG 0B
376101232840 8 days ago /bin/sh -c /tmp/tmp.sh 65.4MB
6c58dda60477 8 days ago /bin/sh -c #(nop) COPY file:a 1.27kB
1264065f6ae8 7 months ago 4.72kB
<missing> 7 months ago 207MB
But after updating the image version, this is the output:
IMAGE CREATED CREATED BY SIZE
3233036cf707 41 hours ago /bin/sh -c #(nop) 0B
1e72b109fe29 41 hours ago /bin/sh -c /tmp/tmp.sh 65.5MB
e4bb0f8240aa 41 hours ago /bin/sh -c #(nop) COPY file:a 1.27kB
dea12a7906f5 12 days ago /bin/sh -c rm -f /tmp/tls-ca- 207MB
<missing> 12 days ago /bin/sh -c rm -f '/etc/yum.re' 0B
<missing> 12 days ago /bin/sh -c #(nop) LABEL "dist 0B
<missing> 12 days ago /bin/sh -c #(nop) ADD file:7 0B
<missing> 12 days ago /bin/sh -c #(nop) ADD file:ad 0B
<missing> 12 days ago /bin/sh -c #(nop) LABEL relea 0B
<missing> 12 days ago /bin/sh -c mkdir -p /var/log 0B
<missing> 12 days ago /bin/sh -c rm -rf /var/log/ 0B
<missing> 12 days ago /bin/sh -c #(nop) CMD ["/bin 0B
<missing> 12 days ago /bin/sh -c #(nop) ENV PATH / 0B
<missing> 12 days ago /bin/sh -c #(nop) ENV contain 0B
<missing> 12 days ago /bin/sh -c #(nop) LABEL io. 0B
<missing> 12 days ago /bin/sh -c #(nop) LABEL io. 0B
<missing> 12 days ago /bin/sh -c #(nop) LABEL io. 0B
<missing> 12 days ago /bin/sh -c #(nop) LABEL des 0B
<missing> 12 days ago /bin/sh -c #(nop) LABEL sum 0B
<missing> 12 days ago /bin/sh -c #(nop) LABEL com 0B
<missing> 12 days ago /bin/sh -c #(nop) LABEL com 0B
<missing> 12 days ago /bin/sh -c #(nop) LABEL 0B
<missing> 12 days ago /bin/sh -c #(nop) ADD multi:3 0B
<missing> 12 days ago /bin/sh -c #(nop) ADD file:21 0B
<missing> 12 days ago /bin/sh -c #(nop) ADD file:09 0B
Any clue why this may be happening? These extra layers that are being added seem to come from the base image. Any clue how to make them appear as a single layer like shown in the older build?
Thanks in advance!

Answer 1

So I realized a couple of things regarding this question at a later point of time.
First off, docker history does not give you the true layer count*. To find the true layer count, I used docker inspect --format='{{join .RootFS.Layers "\n"}} $INSTANCE_ID | wc -l to get the number of true layers.
Secondly, if you still need to squash layers, explore using the --squash directive in docker build (experimental feature), or use docker-squash.
* - The output of docker history shows the list of hashes mapped to the diff of layers. Since these hashes are randomly generated, the mapping is handled by the docker engine. When an image is pulled, this mapping is lost and you get the <missing> value in the IMAGE field. Couldn't have figured this out without this amazing blog post