ci: pin Dockerfile images with sha256

Signed-off-by: Filip Szufnarowski <filip.szufnarowski@intel.com>

Dockerfile

@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 # Copyright (c) 2022-2023 Dell Inc, or its subsidiaries.
 
-FROM docker.io/library/golang:1.21.0 as builder
+FROM docker.io/library/golang:1.21.0@sha256:b490ae1f0ece153648dd3c5d25be59a63f966b5f9e1311245c947de4506981aa as builder
 
 WORKDIR /app
 
@@ -18,7 +18,7 @@ COPY pkg/ pkg/
 RUN go build -v -o /opi-intel-bridge ./cmd/...
 
 # second stage to reduce image size
-FROM alpine:3.18
+FROM alpine:3.18@sha256:7144f7bab3d4c2648d7e59409f15ec52a18006a128c733fcff20d3a4a54ba44a
 COPY --from=builder /opi-intel-bridge /
 COPY --from=docker.io/fullstorydev/grpcurl:v1.8.7-alpine /bin/grpcurl /usr/local/bin/
 EXPOSE 50051