Adding Bridges code as git subtree. (#2515)

* Add instructions.

* Squashed 'bridges/' content from commit 345e84a21

git-subtree-dir: bridges
git-subtree-split: 345e84a2146b56628e9888c9f5e129cb40e868a9

* Remove bridges workspace file to avoid confusing Cargo.

* Add some bridges primitives to Polkadot workspace.

* Improve docs.

polkadot/bridges/Dockerfile

@@ -0,0 +1,71 @@
+# Builds images used by the bridge.
+#
+# In particular, it can be used to build Substrate nodes and bridge relayers. The binary that gets
+# built can be specified with the `PROJECT` build-arg. For example, to build the `substrate-relay`
+# you would do the following:
+#
+# `docker build . -t local/substrate-relay --build-arg=PROJECT=substrate-relay`
+#
+# See the `deployments/README.md` for all the available `PROJECT` values.
+
+# This first stage prepares our dependencies to be built by `cargo-chef`.
+FROM rust as planner
+WORKDIR /parity-bridges-common
+RUN cargo install cargo-chef --version 0.1.13
+COPY . .
+RUN cargo chef prepare --recipe-path recipe.json
+
+# This second stage is where the dependencies actually get built.
+# The reason we split it from the first stage is so that the `COPY . .`
+# step doesn't blow our cache.
+FROM paritytech/bridge-dependencies AS cacher
+WORKDIR /parity-bridges-common
+RUN cargo install cargo-chef --version 0.1.13
+
+COPY --from=planner /parity-bridges-common/recipe.json recipe.json
+RUN cargo chef cook --release --recipe-path recipe.json
+
+# In this third stage we go ahead and build the actual binary we want.
+# This should be fairly quick since the dependencies are being built and
+# cached in the previous stage.
+FROM paritytech/bridge-dependencies as builder
+WORKDIR /parity-bridges-common
+RUN cargo install cargo-chef --version 0.1.13
+
+COPY . .
+COPY --from=cacher /parity-bridges-common/target target
+COPY --from=cacher $CARGO_HOME $CARGO_HOME
+
+ARG PROJECT=ethereum-poa-relay
+RUN cargo build --release --verbose -p ${PROJECT}
+RUN strip ./target/release/${PROJECT}
+
+# In this final stage we copy over the final binary and do some checks
+# to make sure that everything looks good.
+FROM ubuntu:xenial as runtime
+
+# show backtraces
+ENV RUST_BACKTRACE 1
+
+RUN set -eux; \
+	apt-get update && \
+	apt-get install -y libssl-dev curl
+
+RUN groupadd -g 1000 user \
+  && useradd -u 1000 -g user -s /bin/sh -m user
+
+# switch to non-root user
+USER user
+
+WORKDIR /home/user
+
+ARG PROJECT=ethereum-poa-relay
+
+COPY --chown=user:user --from=builder /parity-bridges-common/target/release/${PROJECT} ./
+COPY --chown=user:user --from=builder /parity-bridges-common/deployments/local-scripts/bridge-entrypoint.sh ./
+
+# check if executable works in this container
+RUN ./${PROJECT} --version
+
+ENV PROJECT=$PROJECT
+ENTRYPOINT ["/home/user/bridge-entrypoint.sh"]