add support for generating a docker container

.dockerignore

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+/target

Dockerfile

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+# This Dockerfile is composed of two steps: the first one builds the release
+# binary, and then the binary is copied inside another, empty image.
+
+#################
+#  Build image  #
+#################
+
+FROM ubuntu:bionic AS build
+
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y \
+    ca-certificates \
+    curl \
+    build-essential \
+    pkg-config \
+    libssl-dev
+
+# Install the currently pinned toolchain with rustup
+COPY rust-toolchain /tmp/rust-toolchain
+RUN curl https://static.rust-lang.org/rustup/dist/x86_64-unknown-linux-gnu/rustup-init >/tmp/rustup-init && \
+    chmod +x /tmp/rustup-init && \
+    /tmp/rustup-init -y --no-modify-path --default-toolchain $(cat /tmp/rust-toolchain)
+ENV PATH=/root/.cargo/bin:$PATH
+
+# Build the dependencies in a separate step to avoid rebuilding all of them
+# every time the source code changes. This takes advantage of Docker's layer
+# caching, and it works by copying the Cargo.{toml,lock} with dummy source code
+# and doing a full build with it.
+WORKDIR /tmp/source
+COPY Cargo.lock Cargo.toml /tmp/source/
+COPY parser/Cargo.toml /tmp/source/parser/Cargo.toml
+RUN mkdir -p /tmp/source/src /tmp/source/parser/src && \
+    echo "fn main() {}" > /tmp/source/src/main.rs && \
+    touch /tmp/source/parser/src/lib.rs
+RUN cargo fetch
+RUN cargo build --release
+
+# Dependencies are now cached, copy the actual source code and do another full
+# build. The touch on all the .rs files is needed, otherwise cargo assumes the
+# source code didn't change thanks to mtime weirdness.
+RUN rm -rf /tmp/source/src /tmp/source/parser/src
+COPY src /tmp/source/src
+COPY parser/src /tmp/source/parser/src
+RUN find -name "*.rs" -exec touch {} \; && cargo build --release
+
+##################
+#  Output image  #
+##################
+
+FROM ubuntu:bionic AS binary
+
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y \
+    ca-certificates
+
+COPY --from=build /tmp/source/target/release/triagebot /usr/local/bin/
+CMD ["triagebot"]

rust-toolchain

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+nightly-2019-03-15