Version '1:7.7+19' for 'xserver-xorg' was not found: `xserver-xorg` is outdated and to be replaced by `xserver-xorg-core`

[wallem89]

When trying to balena push <fleet-name> is get the following error:

[main]     Package xserver-xorg is not available, but is referred to by another package.
[main]     This may mean that the package is missing, has been obsoleted, or
[main]     is only available from another source
[main]     However the following packages replace it:
[main]       xserver-xorg-core
[main]     apt failed, retrying
[main]     E: Version '1:7.7+19' for 'xserver-xorg' was not found

When replacing xserver-xorg=1:7.7+19 on line 4 in Dockerfile.template with xserver-xorg-core it works again!

Although I have only get it to work after also changing the debian release as mention in [issue 12](The repository 'http://deb.debian.org/debian stretch Release' does not have a Release file. Had to replace stretch-run with bookwork #12).