Add author/homepage/license to version + banner; date format fixes

- globals.go: add Homepage and License constants.
- version command: show author, homepage, license, build date.
- Startup banner reformatted to:
    vaultik X by Author (commit Y, built on Z) starting up at T.
    https://sneak.berlin/go/vaultik
- Commit date now formatted as YYYY-MM-DD (called "build date" in
  user-facing output, since the binary was at least compiled once on
  the date of commit). Makefile/Dockerfile use git --format=%cs.
  goreleaser slices its RFC3339 .CommitDate template var to 10 chars.

internal/cli/version.go

@@ -17,9 +17,13 @@ func NewVersionCommand() *cobra.Command {
 		Args:  cobra.NoArgs,
 		Run: func(cmd *cobra.Command, args []string) {
 			fmt.Printf("vaultik %s\n", globals.Version)
-			fmt.Printf("  commit:  %s\n", globals.Commit)
-			fmt.Printf("  go:      %s\n", runtime.Version())
-			fmt.Printf("  os/arch: %s/%s\n", runtime.GOOS, runtime.GOARCH)
+			fmt.Printf("  commit:     %s\n", globals.Commit)
+			fmt.Printf("  build date: %s\n", globals.CommitDate)
+			fmt.Printf("  go:         %s\n", runtime.Version())
+			fmt.Printf("  os/arch:    %s/%s\n", runtime.GOOS, runtime.GOARCH)
+			fmt.Printf("  author:     %s\n", globals.Author)
+			fmt.Printf("  homepage:   %s\n", globals.Homepage)
+			fmt.Printf("  license:    %s\n", globals.License)
 			if globals.Version == "dev" {
 				fmt.Println()
 				fmt.Println("This is a development build (no version information embedded).")