fix: use multi-stage build for Go tools in release Dockerfile

Alpine's Go package (1.25.8) is too old for oasdiff@latest which now
requires Go >= 1.26. Switch to a multi-stage build that uses the
official golang:1.26-alpine image to compile Go-based tools, then
copies the binaries into the Alpine runtime stage.

Author: daniel-garcia

Dockerfile.goreleaser

@@ -1,26 +1,29 @@
+## Stage 1: Build Go-based tools with the official Go toolchain
+FROM golang:1.26-alpine AS tools
+
+RUN apk --no-cache add git && \
+    go install github.com/bufbuild/buf/cmd/buf@latest && \
+    go install github.com/oasdiff/oasdiff@latest && \
+    go install google.golang.org/protobuf/cmd/protoc-gen-go@latest && \
+    go install google.golang.org/grpc/cmd/protoc-gen-go-grpc@latest
+
+## Stage 2: Runtime image
 FROM alpine:latest
 
 # dockers_v2 passes TARGETPLATFORM as a build arg (e.g. linux/amd64)
 ARG TARGETPLATFORM
 
-# Install necessary tools
+# Install runtime dependencies
 RUN apk --no-cache add \
     ca-certificates \
     git \
     protobuf \
     nodejs \
-    npm
+    npm && \
+    npm install -g @stoplight/spectral-cli
 
-# Install additional tools
-RUN npm install -g @stoplight/spectral-cli
-
-# Install Go-based tools
-RUN apk add --no-cache go && \
-    go install github.com/bufbuild/buf/cmd/buf@latest && \
-    go install github.com/oasdiff/oasdiff@latest && \
-    go install google.golang.org/protobuf/cmd/protoc-gen-go@latest && \
-    go install google.golang.org/grpc/cmd/protoc-gen-go-grpc@latest && \
-    apk del go
+# Copy Go-based tools from builder
+COPY --from=tools /go/bin/ /usr/local/bin/
 
 # Copy the pre-built binary from GoReleaser (dockers_v2 places binaries under $TARGETPLATFORM/)
 COPY ${TARGETPLATFORM}/apx /usr/local/bin/apx