As builds got more complicated, the ability to only access files from one location became quite limiting. With `multi-stage` builds where you can `copy` files from other parts of the Containerfile by adding the `--from` flag and pointing it to the name of another Containerfile stage or a remote image.
The new named build context feature is an extension of this pattern. You can now define additional build contexts when running the build command, give them a name, and then access them inside a Dockerfile the same way you previously did with build stages.
Additional build contexts can be defined with a new `--build-context [name]=[value]` flag. The key component defines the name for your build context and the value can be:
```console
Local directory – e.g. --build-context project2=../path/to/project2/src
HTTP URL to a tarball – e.g. --build-context src=https://example.org/releases/src.tar
Container image – Define with a docker-image:// prefix, e.g. --build-context alpine=docker-image://alpine:3.15, ( also supports docker://, container-image:// )
```
On the Containerfile side, you can reference the build context on all commands that accept the “from” parameter. Here’s how that might look:
```Dockerfile
FROM [name]
COPY --from=[name] ...
RUN --mount=from=[name] …
```
The value of [name] is matched with the following priority order:
* Named build context defined with `--build-context [name]=..`
* Stage defined with `AS [name]` inside Dockerfile
* Remote image `[name]` in a container registry
Added Features
* Pinning images for `FROM` and `COPY`
* Specifying multiple buildcontexts from different projects
and using them with `--from` in `ADD` and `COPY` directive
* Override a Remote Dependency with a Local One.
* Using additional context from external `Tar`
Signed-off-by: Aditya R <arajan@redhat.com>
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