Add an image to build runtime on illumos

[am11]

The image is built using illumos sysroot from dotnet/arcade, to build products from dotnet/runtime repo.

$ apt install powershell
$ pwsh -c '.\build.ps1 -DockerfilePath "src/ubuntu/18.04/coredeps"'
$ pwsh -c '.\build.ps1 -DockerfilePath "src/ubuntu/18.04/crossdeps"'
$ pwsh -c '.\build.ps1 -DockerfilePath "src/ubuntu/18.04/cross/illumos"'
$ docker history mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-18.04-cross-illumos-20200602195605-6d5e290
IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT
68c74b23daf9        9 hours ago         /bin/sh -c #(nop) ADD file:5de8ed714224eb70a…   561MB
d306e647904b        10 hours ago        /bin/sh -c apt-get update     && apt-get ins…   895MB
fa9b26a00244        10 hours ago        /bin/sh -c apt-get update     && apt-get ins…   596MB
e005c6da8f75        10 hours ago        /bin/sh -c apt-get update     && apt-get ins…   166MB
6a7da23c946f        10 hours ago        /bin/sh -c apt-get update     && apt-get ins…   464MB
1a1778bdcf68        10 months ago       /bin/sh -c #(nop)  CMD ["pwsh"]                 0B
<missing>           10 months ago       /bin/sh -c #(nop)  LABEL maintainer=PowerShe…   0B
<missing>           10 months ago       /bin/sh -c #(nop)  ARG IMAGE_NAME=mcr.micros…   0B
<missing>           10 months ago       /bin/sh -c #(nop)  ARG VCS_REF=none             0B
<missing>           10 months ago       |3 PS_PACKAGE=powershell_6.2.1-1.ubuntu.18.0…   222MB
<missing>           10 months ago       /bin/sh -c #(nop)  ENV DOTNET_SYSTEM_GLOBALI…   0B
<missing>           10 months ago       /bin/sh -c #(nop) ADD f49113060ce8c3d68a2555…   57.9MB
<missing>           10 months ago       |3 PS_PACKAGE=powershell_6.2.1-1.ubuntu.18.0…   0B
<missing>           10 months ago       /bin/sh -c #(nop)  ARG PS_PACKAGE_URL=https:…   0B
<missing>           10 months ago       /bin/sh -c #(nop)  ARG PS_PACKAGE=powershell…   0B
<missing>           10 months ago       /bin/sh -c #(nop)  ARG PS_VERSION=6.2.0         0B
<missing>           11 months ago       /bin/sh -c #(nop)  CMD ["/bin/bash"]            0B
<missing>           11 months ago       /bin/sh -c mkdir -p /run/systemd && echo 'do…   7B
<missing>           11 months ago       /bin/sh -c set -xe   && echo '#!/bin/sh' > /…   745B
<missing>           11 months ago       /bin/sh -c [ -z "$(apt-get indextargets)" ]     987kB
<missing>           11 months ago       /bin/sh -c #(nop) ADD file:4e6b5d9ca371eb881…   63.2MB

Also optimized the dotnet/arcade clone command to make it fetch depth=1 for one branch (instead of all branches).

Contributes to dotnet/runtime#34944.

[am11]

cc @janvorli, @jclulow

Failure seems unrelated to PR changes: Unable to find image info data for path 'src/windowsservercore/ltsc2019/helix/amd64/Dockerfile'.

[mthalman]

Yes, the Unable to find image info data for path 'src/windowsservercore/ltsc2019/helix/amd64/Dockerfile' error is an infrastructure issue and can be ignored.

[MichaelSimons]

This should be defined as a symbolic file link not a copy of the file. This is how the other post-build references are defined - https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/master/src/ubuntu/18.04/cross/arm64-alpine/hooks/post-build

[am11]

I just followed how it was done for FreeBSD: https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/772abb5ae3b7d57d9ed26313c2f5de1f6d330ca8/src/ubuntu/18.04/cross/freebsd/12/hooks/post-build (also v11).

[MichaelSimons]

I see - I guess that means there are multiple places that are incorrect.

[am11]

Fixed by #327.