Use vertical-pod-autoscaler directory as root for each image build

[plkokanov]

What this PR does / why we need it:
The Dockerfiles in the corresponding vpa directories (recommender, updater and admission-controller) are written so that vertical-pod-autoscaler has to be used as work directory:

autoscaler/vertical-pod-autoscaler/pkg/recommender/Dockerfile

Line 25 in 85cd617

RUN CGO_ENABLED=0 LD_FLAGS=-s GOARCH=$TARGETARCH GOOS=$TARGETOS go build -C pkg/recommender -mod vendor -o recommender-$TARGETARCH

Notice the go build -C pkg/recommender part from this line.

This PR tries to adapt the pipeline_definitions to that

In the corresponding Makefiles the docker command is invoked as follows:

docker buildx build --pull --load --platform linux/$* -t ${REGISTRY}/${FULL_COMPONENT}-$*:${TAG} -f ./Dockerfile ../../

This command is called from e.g. vertical-pod-autoscaler/pkg/updater, so notice the ../../ at the end

Which issue(s) this PR fixes:
None

Special notes for your reviewer:

Release note:

NONE

[AndreasBurger]

lgtm