docker: Don't require rebuilding if unnecessary

docker: Don't require rebuilding if unnecessary

Now that we're pinning versions, we're going to see more frequent
dependency version changes. Requiring a rebuild after every one of these
is tiresome so don't force it and instead display a helpful message
merely suggesting that a rebuild may be necessary.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Cc: Daniel Axtens <dja@axtens.net>
Acked-by: Daniel Axtens <dja@axtens.net>
[dja: we're not currently pinning versions because I dropped that
 patch as it broke postgres. But this can stay - sfin has been
 asking for it for ages.]
Signed-off-by: Daniel Axtens <dja@axtens.net>