ignorefiles: cleanup
**- What I did**
Clean up .gitignore based on current tooling/usage of the project.

`.gopath` has been removed; however, I believe that power users who prefer a GOPATH (vs. go mod) based IDE integration are better served by a GOPATH outside the project tree; local ignores can still be established in `.git/info/exclude` as well.

.gitignore
# Docker project generated files to ignore
#  if you want to ignore files created by your editor/tools,
#  please consider a global .gitignore https://help.github.com/articles/ignoring-files
*.exe
*.exe~
*.gz
*.orig
test.main
.*.swp
.DS_Store
# a .bashrc may be added to customize the build environment
.bashrc
.editorconfig
.gopath/
.go-pkg-cache/
bundles/
cli/winresources/**/winres.json
cli/winresources/**/*.syso
cmd/dockerd/dockerd
contrib/builder/rpm/*/changelog
vendor/pkg/
go-test-report.json
profile.out
junit-report.xml

# top-level go.mod is not meant to be checked in
/go.mod

# If you want to ignore files created by your editor/tools, please consider a
# [global .gitignore](https://help.github.com/articles/ignoring-files).

*~
*.bak
*.orig
.*.swp
.DS_Store
thumbs.db

# local repository customization
.envrc
.bashrc
.editorconfig

# top-level go.mod is not meant to be checked in
/go.mod
# build artifacts
bundles/
cli/winresources/*/*.syso
cli/winresources/*/winres.json
contrib/builder/rpm/*/changelog

# ci artifacts
*.exe
*.gz
go-test-report.json
junit-report.xml
profile.out
test.main

This is not the standard location the binary is built (using the makefile), so I guess this was added for cases where someone would manually build it.

If so, perhaps we need to add `cmd/docker-proxy/docker-proxy`

Hm... although we wouldn't know where the binary would land (running `go build ./cmd/docker-proxy/` would put the binary at the root. So perhaps be "less" specific for this one as well, and use `dockerd`, `docker-proxy` ?


Do we need to account for the `libnetwork/cmd/*` directories? Looks like libnetwork itself didn't (at least not for all of them), but did have some other ignores (possibly related to tests?); https://github.com/moby/libnetwork/blob/master/.gitignore