added arm dockerfiles [ci skip]

docker/Dockerfile.linux.arm6

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+FROM arm32v6/alpine:3.6
+RUN apk add --no-cache ca-certificates git openssh curl perl
+
+ADD posix/* /usr/local/bin/
+ENTRYPOINT ["/usr/local/bin/clone"]

docker/Dockerfile.linux.arm7

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+FROM arm32v6/alpine:3.6
+RUN apk add --no-cache ca-certificates git openssh curl perl
+
+ADD posix/* /usr/local/bin/
+ENTRYPOINT ["/usr/local/bin/clone"]

docker/Dockerfile.linux.arm8

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+FROM arm64v8/alpine:3.6
+RUN apk add --no-cache ca-certificates git openssh curl perl
+
+ADD posix/* /usr/local/bin/
+ENTRYPOINT ["/usr/local/bin/clone"]