go build ./... will build the current directory and all subdirectories underneath it.
... is a wildcard indicator for the go tool documented here https://golang.org/cmd/go/:
An import path is a pattern if it includes one or more "..."
wildcards, each of which can match any string, including the empty
string and strings containing slashes. Such a pattern expands to all
package directories found in the GOPATH trees with names matching the
To make common patterns more convenient, there are two special cases.
/... at the end of the pattern can match an empty string, so
net/... matches both net and packages in its subdirectories, like
net/http. Second, any slash-separated pattern element containing a
wildcard never participates in a match of the "vendor" element in the
path of a vendored package, so that
./... does not match packages in
./mycode/vendor/... do. Note, however, that a directory named vendor
that itself contains code is not a vendored package: cmd/vendor would
be a command named vendor, and the pattern cmd/... matches it. See
golang.org/s/go15vendor for more about vendoring.