Add a Dockerfile for generating manpages on s390x

Untested.

Hoping CI and @michael-holzheu can test...

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

man/Dockerfile.s390x

@@ -0,0 +1,25 @@
+FROM    s390x/ubuntu:xenial
+
+RUN     apt-get update && apt-get install -y git golang-go
+
+RUN     mkdir -p /go/src /go/bin /go/pkg
+ENV     GOPATH=/go:/usr/lib/go-1.6
+RUN     export GLIDE=v0.11.1; \
+        export TARGET=/go/src/github.com/Masterminds; \
+        mkdir -p ${TARGET} && \
+        git clone https://github.com/Masterminds/glide.git ${TARGET}/glide && \
+        cd ${TARGET}/glide && \
+        git checkout $GLIDE && \
+        make build && \
+        cp ./glide /usr/bin/glide && \
+        cd / && rm -rf /go/src/* /go/bin/* /go/pkg/*
+
+COPY    glide.yaml /manvendor/
+COPY    glide.lock /manvendor/
+WORKDIR /manvendor/
+RUN     glide install && mv vendor src
+ENV     GOPATH=$GOPATH:/go/src/github.com/docker/docker/vendor:/manvendor
+RUN     go build -o /usr/bin/go-md2man github.com/cpuguy83/go-md2man
+
+WORKDIR /go/src/github.com/docker/docker/
+ENTRYPOINT ["man/generate.sh"]