feat: Add container to propagate host network MTU (#1201)

* feat: Add container to propagate host network MTU

Some network environments use non-standard MTU values. In these
situations, the `DockerMTU` setting might be used to specify the MTU
setting for the `bridge` network created by Docker. However, when the
Github Actions workflow creates networks, it doesn't propagate the
`bridge` network MTU which can lead to `connection reset by peer`
messages.

To overcome this, I've created a new docker image called
`summerwind/actions-runner-mtu` that shims the docker binary in order to
propagate the MTU setting to networks created by Github workflows.

This is a follow-up on the discussion in
(#1046)[https://github.com/actions-runner-controller/actions-runner-controller/issues/1046]
and uses a separate image since there might be some unintended
side-effects with this approach.

* fixup! feat: Add container to propagate host network MTU

Co-authored-by: Yusuke Kuoka <ykuoka@gmail.com>

runner/actions-runner.dockerfile

@@ -60,7 +60,7 @@ RUN set -vx; \
     && if [ "$ARCH" = "amd64" ] || [ "$ARCH" = "i386" ]; then export ARCH=x86_64 ; fi \
     && curl -f -L -o docker.tgz https://download.docker.com/linux/static/${DOCKER_CHANNEL}/${ARCH}/docker-${DOCKER_VERSION}.tgz \
     && tar zxvf docker.tgz \
-    && install -o root -g root -m 755 docker/docker /usr/local/bin/docker \
+    && install -o root -g root -m 755 docker/docker /usr/bin/docker \
     && rm -rf docker docker.tgz \
     && adduser --disabled-password --gecos "" --uid 1000 runner \
     && groupadd docker \
@@ -119,6 +119,10 @@ RUN mkdir /opt/hostedtoolcache \
 # override them with scripts of the same name placed in `/usr/local/bin`.
 COPY entrypoint.sh logger.bash update-status /usr/bin/
 
+# Copy the docker shim which propagates the docker MTU to underlying networks
+# to replace the docker binary in the PATH.
+COPY docker-shim.sh /usr/local/bin/docker
+
 # Configure hooks folder structure.
 COPY hooks /etc/arc/hooks/