fix: preserve source paths in local build context

[ducminhle]

Previously buildx_context_local copied file inputs using basename, which flattened nested files into the context root. This broke Dockerfile COPY paths.

Copy files using their package-relative path so local build contexts keep the expected directory structure.

Example build container image from nodejs source:
Dockerfile

FROM node:24-alpine AS builder

WORKDIR /app

COPY --from=web_src package.json /app/package.json

RUN npm install

COPY --from=web_src . ./

RUN npm run build

FROM nginxinc/nginx-unprivileged:1.30-alpine3.23-slim as web

USER nginx

COPY --from=web_src nginx/conf.d/default.conf /etc/nginx/conf.d/default.conf

COPY --from=builder /app/dist /usr/share/nginx/html

EXPOSE 8080

---

Changes are visible to end-users: yes/no

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: no

Test plan

• Covered by existing test cases

[ducminhle]

I noticed that _copy_sources_to_context_impl was reworked in #14. Did you have any particular ideas or design considerations in mind when you implemented it this way, or am I missing something, @njlr?

[njlr]

I noticed that _copy_sources_to_context_impl was reworked in #14. Did you have any particular ideas or design considerations in mind when you implemented it this way, or am I missing something, @njlr?

Probably just an oversight on my part; your change seems like a good one. Now is a good time to decide this behaviour since the rules are still pre-release. 👍

I'm not a maintainer BTW