providers/docker: more robustly detect built image [GH-4598]

CHANGELOG.md

@@ -7,6 +7,8 @@ BUG FIXES:
   - providers/docker: Nicer error if package is called. [GH-4595]
   - providers/docker: Host IP restriction is forwarded through. [GH-4505]
   - providers/docker: Protocol is now honored in direct `ports settings.
+  - providers/docker: Images built using `build\_dir` will more robustly
+      capture the final image. [GH-4598]
   - providers/virtualbox: Show a human-friendly error if VirtualBox didn't
       clean up an existing VM. [GH-4681]
   - provisioners/docker: Search for docker binary in multiple places. [GH-4580]

plugins/providers/docker/driver.rb

@@ -18,15 +18,15 @@ module VagrantPlugins
         args   = Array(opts[:extra_args])
         args   << dir
         result = execute('docker', 'build', *args)
-        regexp = /Successfully built (.+)$/i
+        matches = result.scan(/Successfully built (.+)$/i)
-        match  = regexp.match(result)
+        if matches.empty?
-        if !match
           # This will cause a stack trace in Vagrant, but it is a bug
           # if this happens anyways.
           raise "UNKNOWN OUTPUT: #{result}"
         end
 
-        match[1]
+        # Return the last match, and the capture of it
+        matches[-1][0]
       end
 
       def create(params, **opts, &block)