Add golangci-lint to our CI

This MR adds golangci-lint to our CI pipeline and fixes current offences.

This iteration includes these linters:

• errcheck: Errcheck is a program for checking for unchecked errors in go programs. These unchecked errors can be critical bugs in some cases
• deadcode: Finds unused code
• ineffassign: Detects when assignments to existing variables are not used
• structcheck: Finds unused struct fields
• typecheck: Like the front-end of a Go compiler, parses and type-checks Go code
• varcheck: Finds unused global variables and constants

Enabling all of the default linters wasn't working correctly with the default docker image, so I've decided to ship MVP first.

Current offences

$ golangci-lint run
integration_test.go:45:17: Error return value of `json.Unmarshal` is not checked (errcheck)
                json.Unmarshal([]byte(gci), &gitalyConnInfo)
                              ^
integration_test.go:129:21: Error return value of `client.DeleteIndex` is not checked (errcheck)
                client.DeleteIndex()
                                  ^
elastic/client_test.go:213:10: Error return value of `w.Write` is not checked (errcheck)
                w.Write([]byte(`{}`))
                       ^
elastic/client_test.go:350:10: Error return value of `w.Write` is not checked (errcheck)
                w.Write([]byte(`{}`))
                       ^
git/repository_test.go:42:17: Error return value of `json.Unmarshal` is not checked (errcheck)
                json.Unmarshal([]byte(gci), &gitalyConnInfo)
                              ^
indexer/indexer_test.go:215:7: Error return value is not checked (errcheck)
        index(idx)
             ^
elastic/client.go:22:2: `envCorrelationIDKey` is unused (deadcode)
        envCorrelationIDKey = "CORRELATION_ID"
        ^