Add farmerless dev node docker image

[jfrank-summit]

Overview

In order to facilitate developer worksflows and testing, it is useful to have a reproducible farmerless dev node image that can be used in local development environments and in CI pipelines for integration testing. The build follows the same approach as the other container builds in the repository.

Implementation details

• The build stage installs the Rust toolchain plus arch-specific cross-compilers whenever the build and target architectures differ, mirroring the approach used in the other container builds.
• Cargo builds subspace-farmerless-dev-node with -Z build-std and --locked to keep deps reproducible, then copies the resulting binary into a minimal Ubuntu runtime image that exposes /var/subspace as a volume.
• The .dockerignore keeps the context lean to improve cache reuse and CI runtimes.
• Snapshot workflow now includes the farmerless-dev-node image in the docker matrix so it is built, tagged, and scanned alongside the other artifacts.

Code contributor checklist:

• I have read, understood and followed contributing guide

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