Update build component to use "image" input variable
The Sphinx build component currently seems to ignore the image input variable when specified and instead runs with the default "python" docker image. I think adding this line should fix that.
An example of where it uses the wrong image is this job, which was run with the following inputs:
include:
- component: git.ligo.org/computing/gitlab/components/sphinx/build@1.0.0
inputs:
stage: docs
image: containers.ligo.org/lscsoft/bilby/v2-bilby-python39
builder: "html"
sphinx_sourcedir: "docs"
sphinx_outputdir: "public"