docker: add 1meg tmpfs mount to docker containers

Problem: In the near future we would like to run tests in which
the file system has run out of space.

In docker, create a 1 meg tmpfs mount that can be used for this
testing.

Author: chu11

src/test/docker/docker-run-checks.sh

@@ -205,6 +205,7 @@ else
     docker run --rm \
         --workdir=$WORKDIR \
         --volume=$TOP:$WORKDIR \
+        --mount type=tmpfs,destination=/test/tmpfs-1m,tmpfs-size=1048576 \
         ${PLATFORM} \
         $MOUNT_HOME_ARGS \
         -e PLATFORM \

src/test/docker/docker-run-systest.sh

@@ -116,6 +116,7 @@ checks_group "Launching system instance container $NAME" \
     --volume=$TOP:$WORKDIR \
     --volume=/sys/fs/cgroup:/sys/fs/cgroup:ro \
     --tmpfs=/run \
+    --mount=type=tmpfs,destination=/test/tmpfs-1m,tmpfs-size=1048576 \
     --cap-add SYS_PTRACE \
     --name=flux-system-test-$$ \
     --network=host \