CI: rework Github build workflows and Dockerfile

Dockerfile

@@ -1,27 +1,38 @@
 ARG ALPINE_VERSION=3.12
 ARG GO_VERSION=1.15
 
-FROM golang:${GO_VERSION}-alpine${ALPINE_VERSION} AS builder
+FROM golang:${GO_VERSION}-alpine${ALPINE_VERSION} AS base
 RUN apk --update add git
 ENV CGO_ENABLED=0
-ARG GOLANGCI_LINT_VERSION=v1.34.1
-RUN [ "$(uname -m)" != "x86_64" ] || wget -O- -nv https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | sh -s ${GOLANGCI_LINT_VERSION}
 WORKDIR /tmp/gobuild
-COPY .golangci.yml .
 COPY go.mod go.sum ./
 RUN go mod download
+COPY cmd/ ./cmd/
+COPY internal/ ./internal/
+
+FROM base AS test
+ENV CGO_ENABLED=1
+RUN apk --update add g++
+RUN go test -race ./...
+
+FROM base AS lint
+ARG GOLANGCI_LINT_VERSION=v1.34.1
+RUN wget -O- -nv https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | \
+    sh -s -- -b /usr/local/bin ${GOLANGCI_LINT_VERSION}
+COPY .golangci.yml ./
+RUN golangci-lint run --timeout=10m
+
+FROM base AS build
 ARG VERSION=unknown
 ARG BUILD_DATE="an unknown date"
 ARG COMMIT=unknown
-COPY cmd/gluetun/main.go .
+COPY cmd/ ./cmd/
 COPY internal/ ./internal/
-RUN [ "$(uname -m)" != "x86_64" ] || go test ./...
-RUN [ "$(uname -m)" != "x86_64" ] || golangci-lint run --timeout=10m
 RUN go build -trimpath -ldflags="-s -w \
     -X 'main.version=$VERSION' \
     -X 'main.buildDate=$BUILD_DATE' \
     -X 'main.commit=$COMMIT' \
-    " -o entrypoint main.go
+    " -o entrypoint cmd/gluetun/main.go
 
 FROM alpine:${ALPINE_VERSION}
 ARG VERSION=unknown
@@ -126,4 +137,4 @@ RUN apk add -q --progress --no-cache --update openvpn ca-certificates iptables i
     mkdir /gluetun
 # TODO remove once SAN is added to PIA servers certificates, see https://github.com/pia-foss/manual-connections/issues/10
 ENV GODEBUG=x509ignoreCN=0
-COPY --from=builder /tmp/gobuild/entrypoint /entrypoint
+COPY --from=build /tmp/gobuild/entrypoint /entrypoint