Free a build node's disk after every build and refuse work under a floor of free space (issue 1834)

A full / made every check on shanks end 'store never became ready' with the reason erased by --rm.
Builds now remove their local image tag, narrowed source and context clone; a build or check
refuses under 20 GiB free on / and the workspace, saying so; the throwaway store is no longer
started with --rm and a store that does not come up reports its exit code and last 20 log lines.

internal/builder/builder_test.go

@@ -38,6 +38,9 @@ type recorded struct {
 	stamped time.Time
 	// tree is what git names as the tree of the module's directory; empty answers as for a commit.
 	tree string
+	// builtFrom is the files (slash-separated, relative) of the folder the last `docker build` ran in, read as it ran:
+	// the build removes that folder when it ends (novox/hq issue 1834).
+	builtFrom []string
 }
 
 func (r *recorded) run(_ context.Context, dir, name string, args ...string) (string, error) {
@@ -53,6 +56,16 @@ func (r *recorded) run(_ context.Context, dir, name string, args ...string) (str
 			break
 		}
 	}
+	if name == "docker" && len(args) > 0 && args[0] == "build" {
+		r.builtFrom = nil
+		_ = filepath.WalkDir(dir, func(p string, d os.DirEntry, err error) error {
+			if err == nil && !d.IsDir() {
+				rel, _ := filepath.Rel(dir, p)
+				r.builtFrom = append(r.builtFrom, filepath.ToSlash(rel))
+			}
+			return nil
+		})
+	}
 	switch {
 	case name == "git" && isClone:
 		repository := args[len(args)-2]