Docker example using bun (#83040)
### What?
Expanded the `with-docker` example for usage with `bun` and added build
instructions to the README.
### Why?
The existing example supports npm, yarn and pnpm, but not bun.
### How?
- Add a separate Dockerfile.bun placed next to the existing Dockerfile,
which is more sensible for the existing solution.
- Dockerfile.bun uses the official oven/bun base image (different
runtime/tooling than node:18-alpine), so a separate file should be
clearer.
- Existing Docker commands in the README remain unchanged, because
Docker still uses the existing Dockerfile by default.
- The bun example can be built with: `docker build -f Dockerfile.bun -t
nextjs-docker .`
Related to #78465, #82998
---------
Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>
Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
Co-authored-by: Joseph <sephxd1234@gmail.com>
