Separate runc binary version from libcontainer version

Now that the dependency on runc (libcontaienr) code has been reduced
considerably, it is probbaly ok to cut the version dependency between
libcontainer and the runc binary that is supported.

This patch separates the runc binary version from the version of
libcontainer that is defined in go.mod, and updates the documentation
accordingly.

The RUNC_COMMIT variable in the install-runc script is renamed to
RUNC_VERSION to encourage using tagged versions, and the Dockerfile
in contrib is updated to allow building with a custom version.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

contrib/Dockerfile.test

@@ -5,6 +5,10 @@
 # 2.) docker run -it --privileged -v /tmp:/tmp --tmpfs /var/lib/containerd-test containerd-test  bash
 # 3.) $ make binaries install test
 #
+# Use the RUNC_VERSION build-arg to build with a custom version of runc, for example,
+# to build runc v1.0.0-rc93, use:
+#
+# docker build -t containerd-test --build-arg RUNC_VERSION=v1.0.0-rc93 -f Dockerfile.test ../
 
 ARG GOLANG_VERSION=1.15.8
 
@@ -32,8 +36,9 @@ RUN apt-get update && apt-get install -y \
     libseccomp-dev \
   --no-install-recommends
 
-COPY go.mod go.mod
 COPY script/setup/install-runc install-runc
+# Allow overriding the version of runc to install through build-args
+ARG RUNC_VERSION
 ARG GOPROXY=direct
 RUN ./install-runc