Fix issues in Docker instructions
One of the commands in the instructions mentioned executing the file
qt-creator-opensource-linux-x86_64-4.6.2.run, which has the exact version 4.6.2 inside of the name. The latest version of the Docker scripts download version 4.8.1 instead, so the instructions were no longer working.
Recently, the Docker repo script was changed to use the more generic name
qt-creator-opensource-linux-x86_64.run so I'm updating the docs to execute that file instead.
Also, one of the instructions mentioned executing krita by typing
krita, but as the
krita-inst/bin folder at that point is not in the $PATH, it won't work. So I'm changing the instructions to
../appimage-workspace/krita-inst/bin/krita which is where the krita executable is located.