build: update build configuration to use internal metadata package and improve ldflags injection, fix defer resp.Body.Close() usage, replace os.Setenv with t.Setenv in tests, correct error message casing, and adjust Dockerfile and Makefile for cmd/reloader structure

Dockerfile

@@ -23,9 +23,8 @@ COPY go.sum go.sum
 RUN go mod download
 
 # Copy the go source
-COPY main.go main.go
+COPY cmd/ cmd/
 COPY internal/ internal/
-COPY pkg/ pkg/
 
 # Build
 RUN CGO_ENABLED=0 \
@@ -34,10 +33,11 @@ RUN CGO_ENABLED=0 \
     GOPROXY=${GOPROXY} \
     GOPRIVATE=${GOPRIVATE} \
     GO111MODULE=on \
-    go build -ldflags="-s -w -X github.com/stakater/Reloader/pkg/common.Version=${VERSION} \
-         -X github.com/stakater/Reloader/pkg/common.Commit=${COMMIT} \
-         -X github.com/stakater/Reloader/pkg/common.BuildDate=${BUILD_DATE}" \
-        -installsuffix 'static' -mod=mod -a -o manager ./
+    go build -ldflags="-s -w \
+         -X github.com/stakater/Reloader/internal/pkg/metadata.Version=${VERSION} \
+         -X github.com/stakater/Reloader/internal/pkg/metadata.Commit=${COMMIT} \
+         -X github.com/stakater/Reloader/internal/pkg/metadata.BuildDate=${BUILD_DATE}" \
+        -installsuffix 'static' -mod=mod -a -o manager ./cmd/reloader
 
 # Use distroless as minimal base image to package the manager binary
 # Refer to https://github.com/GoogleContainerTools/distroless for more details