(chore) bump go version used for releases to 1.15.10

Makefile

@@ -6,7 +6,7 @@ APPVER := $(shell grep 'version =' cmd/version.go | cut -d \" -f2)
 PWD := $(shell pwd)
 LD_FLAGS := -ldflags="-s -w -X=github.com/sensepost/gowitness/cmd.gitHash=$(V) -X=github.com/sensepost/gowitness/cmd.goVer=$(G)"
 BIN_DIR := build
-DOCKER_GO_VER := 1.15.8# https://github.com/elastic/golang-crossbuild
+DOCKER_GO_VER := 1.15.10# https://github.com/elastic/golang-crossbuild
 DOCKER_RELEASE_BUILD_CMD := docker run --rm -it -v $(PWD):/go/src/github.com/sensepost/gowitness \
 	-w /go/src/github.com/sensepost/gowitness -e CGO_ENABLED=1 \
 	docker.elastic.co/beats-dev/golang-crossbuild:$(DOCKER_GO_VER)