Update docker image build timestamp format (#343)

Golang time parser is not compatible with format rendered by the GNU date,
even though both claim to be RFC3339. This change replaces space separator
betwen date and time with "T" which will allow golang to parse it.
It also switches from `--utc` to `-u` that's supported by both GNU and BSD
date commands.

Makefile

@@ -6,7 +6,7 @@ LABEL ?= $(shell git rev-parse --short HEAD)$(and $(shell git status -s),-dirty-
 # If TAG is not provided set default value
 TAG ?= stellar/account-viewer-v2:$(LABEL)
 # https://github.com/opencontainers/image-spec/blob/master/annotations.md
-BUILD_DATE := $(shell date --utc --rfc-3339=seconds)
+BUILD_DATE := $(shell date -u +%FT%TZ)
 
 docker-build:
 	$(SUDO) docker build --pull --label org.opencontainers.image.created="$(BUILD_DATE)" -t $(TAG) .