Allow to override build date with SOURCE_DATE_EPOCH (#887)

in order to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/ for the definition of this variable.

This date call works with different variants of date.
Also use UTC to be independent of timezone.

Makefile

@@ -2,7 +2,12 @@ GIT_TAG?= $(shell git describe --always --tags)
 BIN = gosec
 FMT_CMD = $(gofmt -s -l -w $(find . -type f -name '*.go' -not -path './vendor/*') | tee /dev/stderr)
 IMAGE_REPO = securego
-BUILD_DATE ?=  $(shell date +%Y-%m-%d)
+DATE_FMT=+%Y-%m-%d
+ifdef SOURCE_DATE_EPOCH
+    BUILD_DATE ?= $(shell date -u -d "@$(SOURCE_DATE_EPOCH)" "$(DATE_FMT)" 2>/dev/null || date -u -r "$(SOURCE_DATE_EPOCH)" "$(DATE_FMT)" 2>/dev/null || date -u "$(DATE_FMT)")
+else
+    BUILD_DATE ?= $(shell date "$(DATE_FMT)")
+endif
 BUILDFLAGS := "-w -s -X 'main.Version=$(GIT_TAG)' -X 'main.GitTag=$(GIT_TAG)' -X 'main.BuildDate=$(BUILD_DATE)'"
 CGO_ENABLED = 0
 GO := GO111MODULE=on go