Add Dockerfile for building OpenRS2 in Drone

Signed-off-by: Graham <gpe@openrs2.dev>

.gitignore

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+.*
+!.git*
+!.mailmap
+*~

Dockerfile

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+FROM debian:10-slim
+ENV LANG C.UTF-8
+COPY adoptopenjdk.gpg /etc/apt/trusted.gpg.d/
+RUN apt-get update && \
+  apt-get install -y apt-transport-https ca-certificates && \
+  echo 'deb https://adoptopenjdk.jfrog.io/adoptopenjdk/deb buster main' > /etc/apt/sources.list.d/adoptopenjdk.list && \
+  apt-get update && \
+  mkdir -p /usr/share/man/man1 && \
+  apt-get install -y --no-install-recommends adoptopenjdk-8-hotspot && \
+  apt-get install -y --no-install-recommends gcc libc6-dev libgl1-mesa-dev libxt-dev maven && \
+  apt-get autoremove --purge -y && \
+  rm -rf /var/lib/apt/lists/*

LICENSE

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+Copyright (c) 2020 OpenRS2 Authors
+
+Permission to use, copy, modify, and/or distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
+FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+PERFORMANCE OF THIS SOFTWARE.

README.md

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+# OpenRS2 Docker Image
+
+## Building
+
+```
+docker build -t registry.openrs2.dev/openrs2-dev .
+```
+
+## Deploying
+
+Pushes must be made via a different hostname to work around the registry's
+[compatibility with nginx authentication][registry-auth-bug].
+
+```
+docker login registry-admin.openrs2.dev
+docker tag registry{-admin,}.openrs2.dev/openrs2-dev
+docker push registry-admin.openrs2.dev/openrs2-dev
+```
+
+## License
+
+OpenRS2 is available under the terms of the [ISC license][isc], which is
+similar to the 2-clause BSD license. The full copyright notice and terms are
+available in the `LICENSE` file.
+
+[registry-auth-bug]: https://github.com/docker/distribution/issues/1028
+[isc]: https://opensource.org/licenses/ISC