CGO_ENABLED=0 on Debian Builds to Make Installing Easier

[dstarner]

Installation is a pain right not because of the linking needed by gcc and cgo. Disabling it should hopefully break install errors that I am seeing elsewhere.

 > [10/11] RUN go install github.com/heroku/log-shuttle/...@latest:
1.060 go: downloading github.com/heroku/log-shuttle v0.22.0
1.781 go: downloading github.com/aws/aws-sdk-go v1.21.9
1.782 go: downloading github.com/bmizerany/aws4 v0.0.0-20141025110357-5fb2e7239626
1.821 go: downloading github.com/heroku/slog v0.0.0-20150110001655-7746152d9340
1.821 go: downloading github.com/pborman/uuid v0.0.0-20150824212802-cccd189d45f7
1.821 go: downloading github.com/rcrowley/go-metrics v0.0.0-20141108142129-dee209f2455f
1.825 go: downloading github.com/pebbe/util v0.0.0-20140716220158-e0e04dfe647c
6.332 go: downloading github.com/jmespath/go-jmespath v0.0.0-20180206201540-c2b33e8439af
10.25 # runtime/cgo
10.25 gcc: error: unrecognized command line option '-m64'