While PPA is the preferred method to install JDK 8 in order to get timely updates and fixes, but if you still want to install without PPA, you can download and install JDK 8 from Ubuntu's pool of Universe packages.

• Download JDK and JRE:

  wget http://security.ubuntu.com/ubuntu/pool/universe/o/openjdk-8/openjdk-8-jdk-headless_8u191-b12-2ubuntu0.18.10.1_amd64.deb
  wget http://security.ubuntu.com/ubuntu/pool/universe/o/openjdk-8/openjdk-8-jdk_8u191-b12-2ubuntu0.18.10.1_amd64.deb
  wget http://security.ubuntu.com/ubuntu/pool/universe/o/openjdk-8/openjdk-8-jre_8u191-b12-2ubuntu0.18.10.1_amd64.deb
  wget http://security.ubuntu.com/ubuntu/pool/universe/o/openjdk-8/openjdk-8-jre-headless_8u191-b12-2ubuntu0.18.10.1_amd64.deb
  

• Install using dpkg:

  sudo dpkg -i openjdk-8-jre-headless_8u191-b12-2ubuntu0.18.10.1_amd64.deb \
  openjdk-8-jre_8u191-b12-2ubuntu0.18.10.1_amd64.deb \
  openjdk-8-jdk_8u191-b12-2ubuntu0.18.10.1_amd64.deb \
  openjdk-8-jdk-headless_8u191-b12-2ubuntu0.18.10.1_amd64.deb
  

• Install dependencies:

  sudo apt install -f
  

If you want to install using PPA, add openjdk-r ppa and install:

sudo add-apt-repository ppa:openjdk-r/ppa
sudo apt update
sudo apt install openjdk-8-jdk openjdk-8-jre

sudo add-apt-repository ppa:openjdk-r/ppa
sudo apt-get update
sudo apt-get install openjdk-8-jdk
sudo update-alternatives --config java
sudo update-alternatives --config javac

I am using the distrubution offered by adoptium, called temurin-8-jdk

mkdir -p /etc/apt/keyrings

wget -O - https://packages.adoptium.net/artifactory/api/gpg/key/public | tee /etc/apt/keyrings/adoptium.asc

echo "deb [signed-by=/etc/apt/keyrings/adoptium.asc] https://packages.adoptium.net/artifactory/deb $(awk -F= '/^VERSION_CODENAME/{print$2}' /etc/os-release) main" | tee /etc/apt/sources.list.d/adoptium.list

apt update 

apt install temurin-8-jdk

Optional to switch from an old repository

update-alternatives --config java # choose new java
update-alternatives --config javac # choose new javac

WARNING: this answer suggest unsupported and dangerous mixing of Debian releases. Follow the advice on your own risk, as it can break the system on upgrades, as explained in http://wiki.debian.org/DontBreakDebian#Don.27t_make_a_FrankenDebian

Package mirror search steps:

1. In the Search package directories search for openjdk-8-jdk. You can see two results:

   • stretch (oldstable) (java): OpenJDK Development Kit (JDK)
   • sid (unstable) (java): OpenJDK Development Kit (JDK)

2. Choose stretch repository

3. Scroll to the Download openjdk-8-jdk section and choose your architecture. For example amd64

4. Now you can see mirrors list and instructions how to install the package via apt:

   You should be able to use any of the listed mirrors by adding a line to your /etc/apt/sources.list like this:

   deb http://security.debian.org/debian-security stretch/updates main

Installation steps:

1. Install software source manager

   apt-get update
   apt-get install software-properties-common
   

2. Add mirror with openjdk-8-jdk

   apt-add-repository 'deb http://security.debian.org/debian-security stretch/updates main'
   apt-get update
   

3. Install openjdk 8

   apt-get install openjdk-8-jdk
   

Note: You can use steps above to find an official Debian mirror with any other package you want to install

The first thing is always see the Debian PTS to have an idea of what options are available to you. For instance, if I go to -

https://tracker.debian.org/pkg/openjdk-8

On the left-hand side you have something called versions - can you see it, it shows -

  stable-bpo: 8u121-b13-1~bpo8+1
     testing: 8u121-b13-3
    unstable: 8u121-b13-4

Now stable-bpo means it is in backports. the backports page describes its function excellently, see https://backports.debian.org/

So you need to add as shared by @Reto as well the backports address to the list of packages that you already have -

deb http://ftp.us.debian.org/debian/ jessie main contrib non-free
deb http://security.debian.org/ jessie/updates main contrib non-free
deb http://ftp.us.debian.org/debian/ jessie-updates main contrib non-free
deb http://ftp.us.debian.org/debian/ jessie-backports main contrib non-free

Now update the index, it should say something like this at the very end -

Reading package lists... Done
Building dependency tree       
Reading state information... Done
6 packages can be upgraded. Run 'apt list --upgradable' to see them.

Don't see the number of packages that can be upgraded as that's a function of how many packages I have, how often I update packages etc. If the four lines are similar to above, that means the index updation went without a hitch, no errors.

Once that is done you could simply do -

$ aptitude search openjdk-8

to get a listing of how many different openjdk-8 packages are listed or not. For instance I'm on testing and there are quite a few here. We also have openjdk-9 as well.

Once you get it , just install as you were trying to install before -

$ sudo aptitude install openjdk-8 

and let aptitude handle all the dependancy resolution. If you still have issues, let us know here.

I've managed to solve it by manually downloading the packages with wget:

RUN wget http://security.debian.org/debian-security/pool/updates/main/o/openjdk-8/openjdk-8-jre-headless_8u212-b03-2~deb9u1_amd64.deb \
&& wget http://security.debian.org/debian-security/pool/updates/main/o/openjdk-8/openjdk-8-jre_8u212-b03-2~deb9u1_amd64.deb \
&& wget http://security.debian.org/debian-security/pool/updates/main/o/openjdk-8/openjdk-8-jdk-headless_8u212-b03-2~deb9u1_amd64.deb \
&& wget http://security.debian.org/debian-security/pool/updates/main/o/openjdk-8/openjdk-8-jdk_8u212-b03-2~deb9u1_amd64.deb

and then install it using the dkpg with the -i --force-all options to install all the required dependencies:

RUN dpkg -i --force-all openjdk-8-jre-headless_8u212-b03-2~deb9u1_amd64.deb openjdk-8-jre_8u212-b03-2~deb9u1_amd64.deb openjdk-8-jdk-headless_8u212-b03-2~deb9u1_amd64.deb openjdk-8-jdk_8u212-b03-2~deb9u1_amd64.deb    

Apparently Debian 10 alias Buster only provides openjdk 11 packages.

In my case this meant using debian 9 alias stretch instead. I must admit it wasn't very clever using latest as a version in the first place.

before:

FROM debian:latest

after:

FROM debian:stretch

Debian stretch is a LTS version supported until 06.2022