fix: strip port/proto suffix in wait.ForSQL URL callback

[maxryl]

Summary

• Since testcontainers-go v0.42.0 (chore!: migrate to moby modules, #3591), wait.ForSQL's URL callback receives the port as network.Port.String() ("<num>/<proto>", e.g. "32769/tcp") instead of the bare numeric port. Our callback at container.go was splicing that directly into the URL path, making pgx parse the dbname as tcp/<configured-db>; every SQL probe failed with database "tcp/<db>" does not exist until the 10 s wait timeout expired with a misleading get state: context deadline exceeded from testcontainers' own container-inspection call.
• Fix: strip the /proto suffix via strings.Cut(port, "/") before formatting the DSN.
• Add container_test.go with a TestMain-scoped container and three tests that cover the exact regression (container comes up, instances are usable, instances are isolated from the template).
• Add a minimal Test workflow (go test -race on push & PR).
• Drive-by: SlogAdapter.Printf was passing its variadic args without ..., flagged by go vet (which -race enables). Forward them correctly so fmt.Sprintf sees the intended values.

Root cause reference

github.com/moby/moby/api@v1.54.2/types/network/port.go:

func (p Port) AppendTo(b []byte) []byte {
    if p.IsZero() { return b }
    return fmt.Appendf(b, "%d/%s", p.num, p.proto.Value())
}

Port.String() uses AppendTo, so the port crossing the callback boundary always carries the proto suffix now.