plugins: add Dockerfile for building images

Add a generic dockerfile for building container images of the sample
plugins. The build command creates statically linked binaries and the
resulting images are minimal, based on scratch base image.

Signed-off-by: Markus Lehtonen <markus.lehtonen@intel.com>

.dockerignore

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+build/

plugins/Dockerfile

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+#   Copyright The containerd Authors.
+
+#   Licensed under the Apache License, Version 2.0 (the "License");
+#   you may not use this file except in compliance with the License.
+#   You may obtain a copy of the License at
+
+#       http://www.apache.org/licenses/LICENSE-2.0
+
+#   Unless required by applicable law or agreed to in writing, software
+#   distributed under the License is distributed on an "AS IS" BASIS,
+#   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#   See the License for the specific language governing permissions and
+#   limitations under the License.
+
+ARG GO_VERSION=1.24
+
+FROM golang:${GO_VERSION}-bullseye AS builder
+ARG PLUGIN
+
+WORKDIR /go/src
+
+# Cache dependencies in a separate layer
+COPY go.mod go.sum .
+RUN --mount=type=cache,target=/go/pkg/mod/ go mod download
+
+# Build
+COPY . .
+
+RUN --mount=type=cache,target=/go/pkg/mod/ \
+    make /go/src/build/bin/${PLUGIN} \
+    GO_BUILD="CGO_ENABLED=0 go build -ldflags '-extldflags=-static'"
+
+# Construct final image
+FROM scratch
+ARG PLUGIN
+
+COPY --from=builder /go/src/build/bin/${PLUGIN} /bin/plugin
+
+ENV NRI_PLUGIN_NAME=${PLUGIN}
+
+ENTRYPOINT ["/bin/plugin"]