Fix setting DockerTag and CommitUnixTimestamp

[leizor]

Also add a warning if either variable remains unset on start-up.

[ywwg]

if you update from head you should be able to fix the lint errors by changing the logging calls

[grafanabot]

Go coverage report:

Click to expand.

File	%
github.com/grafana/influx2cortex/pkg/errorx/errors.go	67.3%
github.com/grafana/influx2cortex/pkg/errorx/http_translator.go	100.0%
github.com/grafana/influx2cortex/pkg/influx/api.go	38.3%
github.com/grafana/influx2cortex/pkg/influx/errors.go	100.0%
github.com/grafana/influx2cortex/pkg/influx/parser.go	84.1%
github.com/grafana/influx2cortex/pkg/influx/recorder.go	91.7%
github.com/grafana/influx2cortex/pkg/remotewrite/client.go	69.1%
github.com/grafana/influx2cortex/pkg/remotewrite/measured_client.go	88.9%
github.com/grafana/influx2cortex/pkg/remotewrite/recorder.go	100.0%
github.com/grafana/influx2cortex/pkg/route/registerer.go	100.0%
github.com/grafana/influx2cortex/pkg/server/middleware/http_auth.go	0.0%
github.com/grafana/influx2cortex/pkg/server/middleware/http_fake_auth.go	0.0%
github.com/grafana/influx2cortex/pkg/server/middleware/instrument.go	6.7%
github.com/grafana/influx2cortex/pkg/server/middleware/logging.go	69.7%
github.com/grafana/influx2cortex/pkg/server/middleware/middleware.go	0.0%
github.com/grafana/influx2cortex/pkg/server/middleware/request_limits.go	81.8%
github.com/grafana/influx2cortex/pkg/server/middleware/response.go	57.1%
github.com/grafana/influx2cortex/pkg/server/middleware/route_matcher.go	0.0%
github.com/grafana/influx2cortex/pkg/server/middleware/tracer.go	13.0%
github.com/grafana/influx2cortex/pkg/server/server.go	83.3%
github.com/grafana/influx2cortex/pkg/tenant/resolver.go	75.6%
github.com/grafana/influx2cortex/pkg/tenant/tenant.go	91.7%
github.com/grafana/influx2cortex/pkg/util/bytereplacer/byte_replacer.go	100.0%
github.com/grafana/influx2cortex/pkg/util/extract_forwarded.go	66.7%
github.com/grafana/influx2cortex/pkg/util/http.go	68.3%
github.com/grafana/influx2cortex/pkg/util/log/experimental.go	0.0%
github.com/grafana/influx2cortex/pkg/util/log/log.go	0.0%
github.com/grafana/influx2cortex/pkg/util/log/rate_limit.go	90.0%
github.com/grafana/influx2cortex/pkg/util/log/wrappers.go	0.0%
github.com/grafana/influx2cortex/pkg/util/push/push.go	60.0%
total	63.1%

Go lint report:

No issues found. 😎