Pin alpine base image version in Dockerfile

[TineoC]

Description

Following up on the work done in #739 to pin gcb-docker-gcloud to a SHA256 digest, we should update the base image in our Dockerfile to a pinned version.

Currently, the Dockerfile uses alpine:latest:

• Dockerfile:L1

Using mutable tags like latest can lead to non-reproducible builds or untracked package version bumps. We should update it to a pinned minor version or a specific SHA256 digest (e.g., alpine:3.20 or a specific digest).

CC: @Prajyot-Parab @Priyankasaggu11929 (@Prajyot @Priyanka)

[Prajyot-Parab]

/assign @Prajyot-Parab