Enable docker cross compile for front & auth

auth/Dockerfile

@@ -1,11 +1,11 @@
-FROM golang:1.25 AS build
+FROM --platform=$BUILDPLATFORM golang:1.25 AS build
 WORKDIR /app
 
 COPY go.mod go.sum ./
 RUN go mod download
 
 COPY . .
-RUN CGO_ENABLED=0 GOOS=linux go build -o /keibi
+RUN CGO_ENABLED=0 GOOS=${TARGETOS:-linux} GOARCH=$TARGETARCH go build -o /keibi
 
 FROM gcr.io/distroless/base-debian11
 WORKDIR /app

front/Dockerfile

@@ -1,4 +1,4 @@
-FROM oven/bun AS builder
+FROM --platform=$BUILDPLATFORM oven/bun AS builder
 WORKDIR /app
 
 # https://github.com/oven-sh/bun/issues/24538