apt.kitware.com: Update repository documentation

Not sure in which project this issue is correct, as it is not just for CMake.

Currently apt.kitware.com tells you, you should do:

sudo apt-get update
sudo apt-get install apt-transport-https ca-certificates gnupg software-properties-common wget
wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | gpg --dearmor - | sudo tee /etc/apt/trusted.gpg.d/kitware.gpg >/dev/null
sudo apt-add-repository 'deb https://apt.kitware.com/ubuntu/ focal main'
sudo apt-get update

It seems this documentation is a bit outdated:

• software-properties-common depends on ca-certificates, so no need to add ca-certificates explicitly
• apt-transport-https is only necessary on Ubuntu 16.04
• apt-get update after apt-add-repository is only necessary on Ubuntu 16.04
• gpg seems to be preinstalled (at least on all Ubuntu Docker images from 16.04-21.04). Furthermore there gpg is it's own package, so gnupg (which includes the package gpg may not be necessary)

Successful Dockerfile test

Got on all Dockerfiles the latest version of CMake (currently 3.20.2)

20.04, 20.10 & 21.04:

FROM "ubuntu:20.04"

ENV DEBIAN_FRONTEND=noninteractive

RUN apt-get update \
 && apt-get -V install -y software-properties-common wget \
 && wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | gpg --dearmor - | tee /etc/apt/trusted.gpg.d/kitware.gpg >/dev/nullsoftware-properties-common \
 && apt-add-repository 'deb https://apt.kitware.com/ubuntu/ focal main' \
 && apt-get -V install -y cmake

18.04:

FROM "ubuntu:18.04"

ENV DEBIAN_FRONTEND=noninteractive

RUN apt-get update \
 && apt-get -V install -y software-properties-common wget \
 && wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | gpg --dearmor - | tee /etc/apt/trusted.gpg.d/kitware.gpg >/dev/nullsoftware-properties-common \
 && apt-add-repository 'deb https://apt.kitware.com/ubuntu/ focal main' \
 && apt-get -V install -y cmake

16.04:

FROM "ubuntu:16.04"

ENV DEBIAN_FRONTEND=noninteractive

RUN apt-get update \
 && apt-get -V install -y apt-transport-https software-properties-common wget \
 && wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | gpg --dearmor - | tee /etc/apt/trusted.gpg.d/kitware.gpg >/dev/nullsoftware-properties-common \
 && apt-add-repository 'deb https://apt.kitware.com/ubuntu/ xenial main' \
 && apt-get update \
 && apt-get -V install -y cmake

Conclusion

The documentation could be adjusted to:

1. If you are using a minimal Ubuntu image or a Docker image, you may need to install the following packages:

   For Ubuntu Bionic Beaver (18.04) and newer:

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

   For Ubuntu Xenial Xerus (16.04):

   sudo apt-get update
   sudo apt-get install apt-transport-https software-properties-common wget

2. Obtain a copy of our signing key:

   wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | gpg --dearmor - | sudo tee /etc/apt/trusted.gpg.d/kitware.gpg >/dev/null

3. Add the repository to your sources list and update.

   For Ubuntu Focal Fossa (20.04):

   sudo apt-add-repository 'deb https://apt.kitware.com/ubuntu/ focal main'

   For Ubuntu Bionic Beaver (18.04):

   sudo apt-add-repository 'deb https://apt.kitware.com/ubuntu/ bionic main'

   For Ubuntu Xenial Xerus (16.04):

   sudo apt-add-repository 'deb https://apt.kitware.com/ubuntu/ xenial main'
   sudo apt-get update

Same for point 4 and point 5 could get an addition, that this is usually not necessary within a Dockerfile