Fix docker-image make target

[pmalek-sumo]

COMMIT doesn't seem to be defined anywhere which causes:

docker build -f scripts/stretch.docker -t "telegraf:" .
invalid argument "telegraf:" for "-t, --tag" flag: invalid reference format
See 'docker build --help'.
make: *** [docker-image] Error 125

whereas commit is defined in https://github.com/influxdata/telegraf/blob/master/Makefile#L10

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[sjwang90]

Thanks for the contribution. We'll take a look at the PR and let you know if we have any comments.

[pmalek-sumo]

Actually I'm not sure if we'd like to keep maintaining this as the make target that is referred in scripts/stretch.docker: go-install doesn't exist in the repo anymore.

It was removed in #7759.

Maybe we should just remove the remnants of the previous build scripts/targets?

[reimda]

Thanks for the PR.

#8051 restored the go-install make target, so this fix is useful.