Add Heroku-20 stack (#154)

This is an initial pass at creating a new Heroku-20 stack, based on
Ubuntu 20.04 ("Focal Fosa").

The process for creating the new stack was:
1. Copy the `heroku-18{,-build}` directories to `heroku-20{,-build}`.
2. Adjust the base image / APT sources to substitute 18.04/bionic
  references with 20.04/focal.
3. Adjust the packages list to account for renamed/removed packages,
   until the build passed.
4. Update documentation/Travis files.

To view the changes compared to Heroku-18, see the original PR,
where the copy and rename changes were made as two separate
commits.

Of note:
* The `python` and `python-dev` packages (which were the Python 2
  versions) no longer exist under Ubuntu 20.04. However there are still
  explicitly named `python2` and `python2-dev` packages - so it is at
  least possible to install Python 2 (if we want).
* Even if we don't explicitly install the `python2` package (this
  initial PR doesn't, it only explicitly installs Python 3), for the
  build image (only) it's pulled in by other packages regardless.
* The `python2` and `python2-dev` packages no longer install a
  binary/symlink for `python`. ie: Users must explicitly call the
  command `python{2,3}` and not `python`, unless we choose to add it
  back for compat (using the `python-is-python2` or
  `python-is-python3` packages).

Fixes W-7429923.

.travis.yml

@@ -8,6 +8,7 @@ env:
   - STACK=cedar-14 IMAGE_TAG=heroku/cedar:14 PRIVATE_IMAGE_TAG=heroku/cedar-private:14
   - STACK=heroku-16 IMAGE_TAG=heroku/heroku:16 PRIVATE_IMAGE_TAG=heroku/heroku-private:16
   - STACK=heroku-18 IMAGE_TAG=heroku/heroku:18 PRIVATE_IMAGE_TAG=heroku/heroku-private:18
+  - STACK=heroku-20 IMAGE_TAG=heroku/heroku:20 PRIVATE_IMAGE_TAG=heroku/heroku-private:20
 script:
 - bin/build.sh $STACK $IMAGE_TAG $IMAGE_TAG-build
 - |

BUILD.md

@@ -6,13 +6,14 @@ To build the stack images locally, run this from the repo root:
 
 For example:
 
-    bin/build.sh heroku-18 heroku/heroku:18 heroku/heroku:18-build
+    ./bin/build.sh heroku-20 heroku/heroku:20 heroku/heroku:20-build
 
 The supported stacks are:
 
 * `cedar-14` (this will fail unless the Ubuntu ESM credentials are set in the local environment)
 * `heroku-16` (will also build a `heroku-16-build` image)
 * `heroku-18` (will also build a `heroku-18-build` image)
+* `heroku-20` (will also build a `heroku-20-build` image)
 
 
 # Releasing Stack Images

README.md

@@ -2,9 +2,10 @@
 
 [![Build Status](https://travis-ci.org/heroku/stack-images.svg?branch=master)](https://travis-ci.org/heroku/stack-images)
 
-This repository holds recipes for building [Heroku stack images](https://devcenter.heroku.com/articles/stack).  The recipes are also rendered into Docker images that are available on Docker Hub:
+This repository holds recipes for building [Heroku stack images](https://devcenter.heroku.com/articles/stack).
+The recipes are also rendered into Docker images that are available on Docker Hub:
 
-* [Heroku-16/Heroku-18 Docker image](https://registry.hub.docker.com/u/heroku/heroku/)
+* [Heroku-{16,18,20} Docker images](https://registry.hub.docker.com/u/heroku/heroku/)
 
 Note: The [Cedar-14 Docker image](https://registry.hub.docker.com/u/heroku/cedar/) is no longer being updated,
 since the Ubuntu ESM agreement requires that the updates we receive from Canonical are not published publicly.

heroku-20-build/.dockerignore

@@ -0,0 +1 @@
+installed-packages.txt

heroku-20-build/Dockerfile

@@ -0,0 +1,4 @@
+FROM heroku/heroku:20
+COPY setup.sh /tmp/setup.sh
+RUN LC_ALL=C DEBIAN_FRONTEND=noninteractive /tmp/setup.sh \
+  && rm -rf /var/lib/apt/lists/*