Docker build failing with "Package 'mongodb' has no installation candidate" - mongodb

Question

Up until this morning, building my container was a smooth process, now I'm running into some issues with apt get -y mongodb.
I have a very simple Dockerfile, relevant code posted since this is all that is needed to make reproduce the issue.
FROM python:2.7
RUN apt-get update && \
apt-get -y install \
gpg \
mongodb \
I've researched some fixes but they seem a little overboard, should I just change my base image? I'm trying to understand the cause behind this error and how to future proof it.
Note, I forgot to put the trace.
Sending build context to Docker daemon 1.354GB
Step 1/7 : FROM python:2.7
2.7: Pulling from library/python
5ae19949497e: Pull complete
ed3d96a2798e: Pull complete
f12136850781: Pull complete
1a9ad5d5550b: Pull complete
6f18049a0455: Pull complete
900d7861659d: Pull complete
536c9e631831: Pull complete
cb522be31b83: Pull complete
d03d3d7b603d: Pull complete
Digest: sha256:ee76dd856d06bdd734e1c3c816b243c984586629708f0a3e32038f94a891a5d8
Status: Downloaded newer image for python:2.7
---> 3edf9092825f
Step 2/7 : RUN apt-get update && apt-get -y install gpg mongodb curl
---> Running in 870a931ec60c
Get:1 http://security.debian.org/debian-security buster/updates InRelease [39.1 kB]
Get:2 http://deb.debian.org/debian buster InRelease [118 kB]
Get:3 http://security.debian.org/debian-security buster/updates/main amd64 Packages [11.9 kB]
Get:4 http://deb.debian.org/debian buster-updates InRelease [46.8 kB]
Get:5 http://deb.debian.org/debian buster/main amd64 Packages [7897 kB]
Fetched 8113 kB in 4s (1805 kB/s)
Reading package lists...
Reading package lists...
Building dependency tree...
Reading state information...
Package mongodb is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'mongodb' has no installation candidate
The command '/bin/sh -c apt-get update && apt-get -y install gpg mongodb curl' returned a non-zero code: 100

Answer 1

I guess you delete the old python:2.7 image and rebuild your dockerfile or just change another pc to do the build. If you use a new image, you can see next:
orange#orange:~$ docker images python:2.7
REPOSITORY TAG IMAGE ID CREATED SIZE
python 2.7 3edf9092825f 8 hours ago 884MB
It shows the image was created 8 hours ago, so the image used is quite different compared the one you used days ago.
From this, if you do a search for python:2.7, you may see it now same with 2.7.16-buster, as you know, debian buster released a few days ago, see this:
2019-07-06: Initial release: 10.0 (press release)
So I guess days ago, the python:2.7 is based on debian stretch, so in new debian release, some packages removed which results in your failure.
Best practice:
You can see there are Simple Tags & Shared Tags in python dockerhub repo, please use Simple Tags which will not always change, don't use Shared Tags which may tag to other image id later, for your case, use python:2.7.16-stretch. In fact this base image is the one you used in the past which tagged as python:2.7, the one now which tagged as python:2.7 is not the one you used in the past.