Docker digest supposed to be immutable, but getting different build steps

Back in January I built a version of my app using ‘FROM node:10.12.0’.

The Semaphore build process logs show this:

d8268e1e433b: Pull complete 
Digest: sha256:00a7fb3df8e94ed24f42c2920f132f06e92ea5ed69b1c5e53c4bb3d20e85a3e2
Status: Downloaded newer image for node:10.12.0
 ---> a2b9536415c2
Step 2/11 : RUN apt-get update
 ---> Running in f9bd6b252e7f
Get:1 http://security.debian.org jessie/updates InRelease [44.9 kB]
Ign http://deb.debian.org jessie InRelease
Get:2 http://deb.debian.org jessie-updates InRelease [145 kB]
Get:3 http://security.debian.org jessie/updates/main amd64 Packages [790 kB]
Get:4 http://deb.debian.org jessie Release.gpg [2420 B]
Get:5 http://deb.debian.org jessie-updates/main amd64 Packages [23.0 kB]
Get:6 http://deb.debian.org jessie Release [148 kB]
Get:7 http://deb.debian.org jessie/main amd64 Packages [9098 kB]
Fetched 10.3 MB in 2s (3476 kB/s)
Reading package lists...

Just now I attempted to put up a new build. I’ve not touched these build files or built since January. I thought everything should work. But I got this instead:

d8268e1e433b: Pull complete 
Digest: sha256:00a7fb3df8e94ed24f42c2920f132f06e92ea5ed69b1c5e53c4bb3d20e85a3e2
Status: Downloaded newer image for node:10.12.0
 ---> a2b9536415c2
Step 2/11 : RUN apt-get update
 ---> Running in e903db31c4a6
Get:1 http://security.debian.org jessie/updates InRelease [44.9 kB]
Ign http://deb.debian.org jessie InRelease
Get:2 http://deb.debian.org jessie-updates InRelease [7340 B]
Get:3 http://deb.debian.org jessie Release.gpg [2420 B]
Get:4 http://deb.debian.org jessie Release [148 kB]
Get:5 http://security.debian.org jessie/updates/main amd64 Packages [825 kB]
Get:6 http://deb.debian.org jessie/main amd64 Packages [9098 kB]
Fetched 10.1 MB in 4s (2509 kB/s)
W: Failed to fetch http://deb.debian.org/debian/dists/jessie-updates/InRelease  Unable to find expected entry 'main/binary-amd64/Packages' in Release file (Wrong sources.list entry or malformed file)

E: Some index files failed to download. They have been ignored, or old ones used instead.
The command '/bin/sh -c apt-get update' returned a non-zero code: 100

So I found out about Docker Digests. Should be able to have purely immutable builds using Digests. But… the digests of these two builds are the same!

Am I correct in thinking using a digest in the ‘FROM’ statement would not have helped me?

How could these two different builds have the same digest?

Source: StackOverflow