check in go.mod/go.sum, add Dockerfile

[derhuerst]

Even though the binaries work great as a low-effort way to run gtfstidy on some GTFS feeds, in some environments (e.g. CIs) using Docker is more straightforward.

still missing:

• add metadata to the Docker image
• let the CI build & publish the Docker image?

---

This is a follow-up PR on #8, which I have accidentally closed.

[derhuerst]

The Docker build currently fails with these errors:

#11 [builder 7/7] RUN CGO_ENABLED=0 go build -o /usr/local/bin/gtfstidy
#11 58.64 # github.com/patrickbr/gtfstidy/processors
#11 58.64 processors/completegeotripsfilter.go:16:24: undefined: gtfsparser.Polygon
#11 58.64 processors/completegeotripsfilter.go:41:28: invalid operation: cannot call non-function st.Stop (variable of type *gtfs.Stop)
#11 58.64 processors/completegeotripsfilter.go:48:9: feed.DeleteTrip undefined (type *gtfsparser.Feed has no field or method DeleteTrip)
#11 58.64 processors/completegeotripsfilter.go:51:15: invalid operation: cannot call non-function st.Stop (variable of type *gtfs.Stop)
#11 58.64 processors/completegeotripsfilter.go:52:8: invalid operation: cannot call non-function st.Stop (variable of type *gtfs.Stop)
#11 58.64 processors/completegeotripsfilter.go:53:16: invalid operation: cannot call non-function st.Stop (variable of type *gtfs.Stop)
#11 58.64 processors/completegeotripsfilter.go:79:9: feed.DeletePathway undefined (type *gtfsparser.Feed has no field or method DeletePathway)
#11 58.64 processors/stopduplicateremover.go:162:34: undefined: gtfs.TransferKey
#11 58.64 processors/agencyduplicateremover.go:126:8: feed.DeleteAgency undefined (type *gtfsparser.Feed has no field or method DeleteAgency)
#11 58.64 processors/agencyduplicateremover.go:170:25: feed.AgenciesAddFlds undefined (type *gtfsparser.Feed has no field or method AgenciesAddFlds)
#11 58.64 processors/completegeotripsfilter.go:79:9: too many errors
#11 ERROR: process "/bin/sh -c CGO_ENABLED=0 go build -o /usr/local/bin/gtfstidy" did not complete successfully: exit code: 1