Add Dockerfile for testing the build

.dockerignore

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+.dockerignore
+/*.sh
+/build
+Dockerfile

Dockerfile

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+# This dockerfile can be used to both build a containerized version of the application
+# and to self-test its build.
+# * To self-test, simply `docker build -t opustags .`; `make check` is run as the final step anyway.
+# * To use the dockerized version, `docker run -it opustags opustags -h` (etc.)
+
+FROM ubuntu:18.04
+RUN apt-get update
+RUN apt-get install --no-install-recommends -y make cmake g++ libogg-dev pkg-config ffmpeg liblist-moreutils-perl locales
+RUN locale-gen en_US.UTF-8
+ENV LANG en_US.UTF-8
+ENV LC_ALL en_US.UTF-8
+ADD . /src
+WORKDIR /build
+RUN env CXX=g++ cmake /src && make && make install
+# We need to run tests as a regular user, since on Docker /dev is a writable directory
+# for root (it's backed by a tmpfs).  This would make the "device as partial file" test fail.
+RUN useradd ubuntu
+RUN mkdir -p /build && chown -R ubuntu:ubuntu /build /src
+USER ubuntu
+RUN env CXX=g++ make check