ci: move cross compile check from travis to gha

In here we have to use Docker, as Ubuntu does not support
all the architectures we're compile-testing here.

Since this is the only step that is using Docker,
there is no sense to separate `make runcimage` from
the rest of it. In case we'll have to use Docker image
more, it will make sense to do so.

While at it, ditch script/tmpmount (added by commit 1735ad788f),
because
 - it required root (because mount);
 - it is probably no longer needed.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Dockerfile

@@ -60,9 +60,7 @@ ARG UMOCI_VERSION
 RUN curl -o /usr/local/bin/umoci -fsSL https://github.com/opencontainers/umoci/releases/download/${UMOCI_VERSION}/umoci.amd64 \
     && chmod +x /usr/local/bin/umoci
 
-COPY script/tmpmount /
 WORKDIR /go/src/github.com/opencontainers/runc
-ENTRYPOINT ["/tmpmount"]
 
 # setup a playground for us to spawn containers in
 COPY tests/integration/multi-arch.bash tests/integration/