Fixed cross-compilation

Dockerfile

@@ -19,18 +19,24 @@
 FROM golang:1.26-alpine AS build
 
 ARG VERSION=dev
+# The platform being built for, so `go build` cross-compiles to it rather than to
+# the builder's own architecture.
+ARG TARGETOS
+ARG TARGETARCH
 WORKDIR /src
 
 # Only what the server is built from, so a change to the clients, the
 # Android app or the docs does not invalidate this layer.
 COPY go.mod go.sum ./
-RUN go mod download
+RUN --mount=type=cache,target=/go/pkg/mod \
+    go mod download
 COPY cmd/keeporgd ./cmd/keeporgd
 COPY internal ./internal
 
-RUN CGO_ENABLED=0 GOOS=linux go build \
-        -trimpath \
-        -ldflags "-s -w -X main.Version=${VERSION}" \
+RUN --mount=type=cache,target=/go/pkg/mod \
+    --mount=type=cache,target=/root/.cache/go-build \
+    CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -trimpath \
+    -ldflags="-s -w -X main.version=${APP_VERSION}" \
         -o /out/keeporgd ./cmd/keeporgd
 
 # Empty directories to copy into the scratch image with the right owner.