fix(Makefile): remove illegal character from docker tag

Author: Aaron Schlesinger

Makefile

@@ -12,7 +12,7 @@ DEV_ENV_CMD := ${DEV_ENV_PREFIX} ${DEV_ENV_IMAGE}
 
 # SemVer with build information is defined in the SemVer 2 spec, but Docker
 # doesn't allow +, so we use -.
-VERSION ?= git-$(shell git rev-parse --short HEAD)
+VERSION ?= git$(shell git rev-parse --short HEAD)
 BINARY_DEST_DIR := rootfs/usr/bin
 # Common flags passed into Go's linker.
 LDFLAGS := "-s -X main.version=${VERSION}"