Support multi-arch Docker builds

Author: jesseslone

DEVELOPMENT.md

@@ -114,6 +114,19 @@ We use a multi-stage Alpine-based build — keeps the final image around ~33MB:
 docker build -f docker/Dockerfile -t louisho5/picobot:latest .
 ```
 
+#### Multi-arch builds with BuildKit
+
+Picobot's Dockerfile supports BuildKit/`buildx` so you can push both AMD64 and ARM64 images in a single run:
+
+```sh
+docker buildx build \
+  --platform linux/amd64,linux/arm64 \
+  --builder default \
+  -t louisho5/picobot:latest .
+```
+
+Add `--push` to publish directly to a registry or `--load` to import one architecture into your local Docker engine.
+
 > **Important:** Run this from the **project root**, not from inside `docker/`. The build context needs access to the whole codebase.
 
 ### Test it locally
@@ -241,4 +254,4 @@ Try cleaning and re-downloading deps:
 go clean -cache
 go mod tidy
 go build ./...
-```
+```

docker/Dockerfile

@@ -1,5 +1,12 @@
+# syntax=docker/dockerfile:1.7
+
 # ---- Build Stage ----
-FROM golang:1.26-alpine AS builder
+ARG BUILDPLATFORM
+FROM --platform=$BUILDPLATFORM golang:1.26-alpine AS builder
+
+ARG TARGETOS
+ARG TARGETARCH
+ARG TARGETVARIANT
 
 RUN apk add --no-cache git ca-certificates tzdata
 
@@ -8,7 +15,13 @@ COPY go.mod go.sum ./
 RUN go mod download
 
 COPY . .
-RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 \
+RUN set -eux; \
+    GOOS="${TARGETOS:-linux}"; \
+    GOARCH="${TARGETARCH:-amd64}"; \
+    if [ "$GOARCH" = "arm" ] && [ -n "${TARGETVARIANT}" ]; then \
+        GOARM="${TARGETVARIANT#v}"; \
+    fi; \
+    CGO_ENABLED=0 GOOS="$GOOS" GOARCH="$GOARCH" ${GOARM:+GOARM=$GOARM} \
     go build -ldflags="-s -w" -o /picobot ./cmd/picobot
 
 # ---- Runtime Stage ----