Add BuildKite agent.

This has no effect on the continuous integration system, and simply
publishes a cached container image containing the agent and metrics
agent with known provenance.

PiperOrigin-RevId: 345274375

images/agent/Dockerfile

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+FROM golang:1.15 as build-agent
+RUN git clone --depth=1 --branch=v3.25.0 https://github.com/buildkite/agent
+RUN cd agent && go build -i -o /buildkite-agent .
+
+FROM golang:1.15 as build-agent-metrics
+RUN git clone --depth=1 --branch=v5.2.0 https://github.com/buildkite/buildkite-agent-metrics
+RUN cd buildkite-agent-metrics && go build -i -o /buildkite-agent-metrics .
+
+FROM gcr.io/distroless/base-debian10
+COPY --from=build-agent /buildkite-agent /
+COPY --from=build-agent-metrics /buildkite-agent-metrics /
+CMD ["/buildkite-agent"]

images/agent/README.md

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+# Build Agent
+
+This is the image used by the build agent. It is built and bundled via a
+separate packaging mechanism in order to provide local caching and to ensure
+that there is better build provenance. Note that continuous integration system
+will generally deploy new agents from the primary branch, and will only deploy
+as instances are recycled. Updates to this image should be made carefully.