test on centos7 via docker

Dockerfile.centos7

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+FROM centos:7
+
+RUN yum install -y make sqlite-devel zlib-devel bash git gcc-c++
+
+# Create a directory and copy in all files
+RUN mkdir -p /tmp/tippecanoe-src
+WORKDIR /tmp/tippecanoe-src
+COPY . /tmp/tippecanoe-src
+
+# Build tippecanoe
+RUN make \
+  && make install
+
+# Run the tests
+CMD make test