classic builder: align "removing intermediate container" output

This is something that stood out to me: removing the intermediate
container is part of a build step, but unlike the other output from
the build, wasn't indented (and prefixed with `--->`) to be shown
as part of the build.

This patch adds the `--->` prefix, to make it clearer what step the
removal was part of.

While at it, I also updated the message itself: this output is printed
_after_ the intermediate container has been removed, so we may as well
make it match reality, so I changed "removing" to "removed".

Before:

    echo -e 'FROM busybox\nRUN echo hello > /dev/null\nRUN echo world > /dev/null\n' | DOCKER_BUILDKIT=0 docker build --no-cache -
    Sending build context to Docker daemon  2.048kB
    Step 1/3 : FROM busybox
     ---> a416a98b71e2
    Step 2/3 : RUN echo hello > /dev/null
     ---> Running in a1a65b9365ac
    Removing intermediate container a1a65b9365ac
     ---> 8c6b57ebebdd
    Step 3/3 : RUN echo world > /dev/null
     ---> Running in 9fa977b763a5
    Removing intermediate container 9fa977b763a5
     ---> 795c1f2fc7b9
    Successfully built 795c1f2fc7b9

After:

    echo -e 'FROM busybox\nRUN echo hello > /dev/null\nRUN echo world > /dev/null\n' | DOCKER_BUILDKIT=0 docker build --no-cache -
    Sending build context to Docker daemon  2.048kB
    Step 1/3 : FROM busybox
     ---> fc9db2894f4e
    Step 2/3 : RUN echo hello > /dev/null
     ---> Running in 38d7c34c2178
     ---> Removed intermediate container 38d7c34c2178
     ---> 7c0dbc45111d
    Step 3/3 : RUN echo world > /dev/null
     ---> Running in 629620285d4c
     ---> Removed intermediate container 629620285d4c
     ---> b92f70f2e57d
    Successfully built b92f70f2e57d

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

builder/dockerfile/containerbackend.go

@@ -141,6 +141,6 @@ func (c *containerManager) RemoveAll(stdout io.Writer) {
 			return
 		}
 		delete(c.tmpContainers, containerID)
-		fmt.Fprintf(stdout, "Removing intermediate container %s\n", stringid.TruncateID(containerID))
+		_, _ = fmt.Fprintf(stdout, " ---> Removed intermediate container %s\n", stringid.TruncateID(containerID))
 	}
 }