Add earthly build configuration for building docs.

I prefer the now acceptable build.earth filename over for the
Filefile-style Earthfile.

Along with the build.earth I've also included a .earthlyignore file
which uses the same format as a [.dockerignore] file.
Although we are not creating distributable images ignoring spurious
files in the docker context helps cut down on cache misses invalidating
dependency layers in the build.

[.dockerignore]: https://docs.docker.com/build/concepts/context/#dockerignore-files

.earthlyignore

@@ -0,0 +1,19 @@
+# https://docs.docker.com/build/concepts/context/#dockerignore-files
+# Unlike .gitignore files directories are not ignored at all layers by default.
+# The **/ prefix will make a pattern behave like a .gitignore pattern and apply
+# in any directory, including the root.
+**/*.pyc
+**/.direnv/
+**/.envrc
+**/.git/
+**/.github/
+**/.pytest_cache/
+**/.ruff_cache/
+**/__pycache__/
+**/_build/
+**/build/
+**/deb_dist/
+**/dist/
+**/rosdep.egg-info/
+**/rosdep_modules.egg-info/
+**/target/

build.earth

@@ -0,0 +1,35 @@
+VERSION 0.8
+
+build:
+  FROM docker.io/library/python:3.12
+  RUN mkdir -p /src || true
+  COPY --dir doc src test \
+    CHANGELOG.rst \
+    Makefile \
+    README.md \
+    rosdoc.yaml \
+    setup.cfg \
+    setup.py \
+    stdeb.cfg \
+    /src/rosdep
+  WORKDIR /src/rosdep
+  RUN pip install -e .
+
+testdeps:
+  FROM +build
+  RUN pip install .[test]
+
+test:
+  # TODO tests aren't passing in context.
+  FROM +testdeps
+  RUN pytest test
+
+docdeps:
+  FROM +build
+  RUN pip install .[doc]
+
+doc:
+  FROM +docdeps
+  WORKDIR /src/rosdep/doc
+  RUN make html
+  SAVE ARTIFACT _build/html AS LOCAL doc/_build/html