Additional documentation on reproducible builds (#872)

The PR adds deterministic build motivation and additional details on to
the risczero build docs, including a link to install the Docker
dependency on desktops.

This resolves #867.

---------

Co-authored-by: Sabree Blackmon <sabree@risczero.com>
Co-authored-by: Tim Zerrell <tim.zerrell@risczero.com>
Co-authored-by: Frank Laub <flaub@risc0.com>

risc0/cargo-risczero/README.md

@@ -14,6 +14,8 @@ After that you can verify it works via:
 ```bash
 cargo risczero --version
 ```
+### Docker
+In order to use the `build` command, you will need `docker` available in your PATH. For developer machines, this is simple with [Docker Desktop.](https://docs.docker.com/desktop/)
 
 ## install
 
@@ -67,15 +69,22 @@ Use the `build-toolchain` command to build the toolchain locally for your host.
 
 ## build
 
-Use the `build` command to build a guest code for the target `riscv32im-risc0-zkvm-elf`. The resulting ELF binary will be saved on `target/riscv-guest/riscv32im-risc0-zkvm-elf/docker/`. Warning: this requires having docker installed.
+Use the `build` command to build guest code for the zkVM target `riscv32im-risc0-zkvm-elf` deterministically.
 
-### Examples
+The compiled ELF is saved in: `./target/riscv-guest/riscv32im-risc0-zkvm-elf/docker/`
 
-```bash
-# Build the factors example
-cargo risczero build --manifest-path examples/factors/methods/guest/Cargo.toml
+With this containerized build process, we ensure that all builds of your guest code, regardless of the machine or local environment, will produce the same ImageID. The ImageID, and its importance to [security,](https://dev.risczero.com/tech_faq#security) is explained in more detail in our [developer FAQ.](https://dev.risczero.com/tech_faq#zkvm-application-design)
+
+Note: The build command requires the docker CLI installed and in your PATH.
 
-# Build the chess example
-cargo risczero build --manifest-path examples/chess/methods/guest/Cargo.toml
+### Example
+
+```bash
+# Build the zkVM's test examples
+cargo risczero build --manifest-path risc0/zkvm/methods/guest/Cargo.toml
 
+ELFs ready at:
+ImageID: 417778745b43c82a20db33a55c2b1d6e0805e0fa7eec80c9654e7321121e97af - "target/riscv-guest/riscv32im-risc0-zkvm-elf/docker/risc0_zkvm_methods_guest/multi_test"
+ImageID: c7c399c25ecf26b79e987ed060efce1f0836a594ad1059b138b6ed2f123dad38 - "target/riscv-guest/riscv32im-risc0-zkvm-elf/docker/risc0_zkvm_methods_guest/hello_commit"
+ImageID: a51a4b747f18b7e5f36a016bdd6f885e8293dbfca2759d6667a6df8edd5f2489 - "target/riscv-guest/riscv32im-risc0-zkvm-elf/docker/risc0_zkvm_methods_guest/slice_io"
 ```