ci: Fix building disabled containers

* Change the context into the disabled directory. Now you can test
  containers which are disabled.

src/ci/docker/disabled/aarch64-gnu/Dockerfile

@@ -31,7 +31,7 @@ WORKDIR /build
 # The `config` config file was a previously generated config file for
 # the kernel. This file was generated by running `make defconfig`
 # followed by `make menuconfig` and then enabling the IPv6 protocol page.
-COPY disabled/aarch64-gnu/config /build/.config
+COPY aarch64-gnu/config /build/.config
 RUN curl https://cdn.kernel.org/pub/linux/kernel/v4.x/linux-4.4.42.tar.xz | \
       tar xJf - && \
       cd /build/linux-4.4.42 && \

src/ci/docker/disabled/wasm32-exp/Dockerfile

@@ -17,7 +17,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 
 # emscripten
 COPY scripts/emscripten-wasm.sh /scripts/
-COPY disabled/wasm32-exp/node.sh /usr/local/bin/node
+COPY wasm32-exp/node.sh /usr/local/bin/node
 RUN bash /scripts/emscripten-wasm.sh
 
 # cache

src/ci/docker/run.sh

@@ -36,12 +36,14 @@ elif [ -f "$docker_dir/disabled/$image/Dockerfile" ]; then
         echo Cannot run disabled images on travis!
         exit 1
     fi
-    retry docker \
+    # retry messes with the pipe from tar to docker. Not needed on non-travis
+    # Transform changes the context of disabled Dockerfiles to match the enabled ones
+    tar --transform 's#^./disabled/#./#' -C $docker_dir -c . | docker \
       build \
       --rm \
       -t rust-ci \
-      -f "$docker_dir/disabled/$image/Dockerfile" \
-      "$docker_dir"
+      -f "$image/Dockerfile" \
+      -
 else
     echo Invalid image: $image
     exit 1