Add check builder for Windows to Travis

src/ci/docker/mingw-check/Dockerfile

@@ -0,0 +1,22 @@
+FROM ubuntu:16.04
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+  g++ \
+  make \
+  file \
+  curl \
+  ca-certificates \
+  python2.7 \
+  git \
+  cmake \
+  sudo \
+  gdb \
+  xz-utils \
+  libssl-dev \
+  pkg-config \
+  mingw-w64
+
+COPY scripts/sccache.sh /scripts/
+RUN sh /scripts/sccache.sh
+
+ENV SCRIPT python2.7 ../x.py check --target=i686-pc-windows-gnu --host=i686-pc-windows-gnu