Revert "Add multi-arch docker image build support"

This reverts commit 9fa50b2.

Pushing of ARM images is not working[1], and there are issues with
pushing master images[2].

Rather than deal with risky fixes with buildx days before the v0.45.0
release, we've decided to roll back this change and migrate to buildx
later, once we've understood and tested the build process better.

[1]: #3015 (comment)

[2]: #3127

.github/workflows/build.yml

@@ -140,14 +140,7 @@ jobs:
         with:
           fetch-depth: 0
       - name: Build
-        run: |
-          docker buildx create \
-            --name multibuilder \
-            --platform linux/amd64,linux/arm64 \
-            --bootstrap --use
-          docker buildx build \
-            --platform linux/amd64,linux/arm64 \
-            -t $DOCKER_IMAGE_ID .
+        run: docker build -t $DOCKER_IMAGE_ID .
       - name: Check
         run: |
             docker run $DOCKER_IMAGE_ID version

Dockerfile

@@ -1,24 +1,14 @@
-# syntax=docker/dockerfile:1
-# Build
-FROM --platform=$BUILDPLATFORM golang:1.20-alpine3.17 AS builder
-
-ARG TARGETOS TARGETARCH
-
-ENV GOOS=$TARGETOS \
-  GOARCH=$TARGETARCH \
-  CGO_ENABLED=0
-
+FROM golang:1.20-alpine3.17 as builder
 WORKDIR $GOPATH/src/go.k6.io/k6
 COPY . .
 RUN apk --no-cache add git=~2
-RUN go build -a -trimpath -ldflags "-s -w -X go.k6.io/k6/lib/consts.VersionDetails=$(date -u +"%FT%T%z")/$(git describe --tags --always --long --dirty)" -o /usr/bin/k6 .
+RUN CGO_ENABLED=0 go install -a -trimpath -ldflags "-s -w -X go.k6.io/k6/lib/consts.VersionDetails=$(date -u +"%FT%T%z")/$(git describe --tags --always --long --dirty)"
 
-# Runtime stage
 FROM alpine:3.17
 # hadolint ignore=DL3018
 RUN apk add --no-cache ca-certificates && \
     adduser -D -u 12345 -g 12345 k6
-COPY --from=builder /usr/bin/k6 /usr/bin/k6
+COPY --from=builder /go/bin/k6 /usr/bin/k6
 
 USER 12345
 WORKDIR /home/k6