build: help dependabot detect base image versions

Signed-off-by: Mark Sagi-Kazar <mark.sagikazar@gmail.com>

Dockerfile

-ARG BASEIMAGE=alpine:3.15.4
+ARG BASEIMAGE=alpine
 
 FROM golang:1.17.8-alpine3.14 AS builder
 
@@ -40,6 +40,9 @@ RUN wget -O /usr/local/bin/gomplate \
     "https://github.com/hairyhenderson/gomplate/releases/download/${GOMPLATE_VERSION}/gomplate_${TARGETOS:-linux}-${TARGETARCH:-amd64}${TARGETVARIANT}" \
     && chmod +x /usr/local/bin/gomplate
 
+# For Dependabot to detect base image versions
+FROM alpine:3.15.4 AS alpine
+FROM gcr.io/distroless/static:latest AS distroless
 
 FROM $BASEIMAGE