libc6.so.6 is in the package
libc6-i386, so install that package too.
libc6.so.6 you are missing is the 32 bit version from
/lib32/libc6.so.6. There is another 64 bit version in
/lib/x86_64-linux-gnu/libc.so.6 but that's not the one you need for Steam. After installing all the missing 32 bit libraries steam needs, you will still need the 32 bit loader, which is also in the
libc6-i386 package. Once the 32 bit loader is present, the
ldd script run on your Steam executable will produce output listing the required libraries, and which are missing.
The 32 bit packages may be distinguished from the 64 bit versions by adding ":i386" to the package name when installing, e.g.:
sudo apt-get install libgl1-mesa-dri:i386
libc6:i386 and the
libc6-386 packages are probably the same. Some people suggest:
sudo dpkg --add-architecture i386
sudo apt-get update
commands, but I've never had to do that.