Append custom env vars instead of replace in Process (#171)

Append custom env vars instead of replace in Process (#168, #169)

PR #162 refactored the default configuration code. This
introduced a subtle bug where `env` became `[]string{}` instead of the
default of `nil`.

In golang, `exec.Cmd.Env == nil` means to use the "current process's
environment". By setting it to `[]string{}` as a default the Process's
environment was emptied out which caused an array of strange and
difficult to troubleshoot behaviour. See issues #168 and #169

This commit changes the behaviour to append model configured environment
variables to the default list rather than replace them.

proxy/process.go

@@ -189,18 +189,19 @@ func (p *Process) start() error {
 	p.waitStarting.Add(1)
 	defer p.waitStarting.Done()
 	cmdContext, ctxCancelUpstream := context.WithCancel(context.Background())
-	p.proxyLogger.Debugf("<%s> Executing start command: %s", p.ID, strings.Join(args, " "))
+
 	p.cmd = exec.CommandContext(cmdContext, args[0], args[1:]...)
 	p.cmd.Stdout = p.processLogger
 	p.cmd.Stderr = p.processLogger
-	p.cmd.Env = p.config.Env
-
+	p.cmd.Env = append(p.cmd.Environ(), p.config.Env...)
 	p.cmd.Cancel = p.cmdStopUpstreamProcess
 	p.cmd.WaitDelay = p.gracefulStopTimeout
 	p.cancelUpstream = ctxCancelUpstream
 	p.cmdWaitChan = make(chan struct{})
 
 	p.failedStartCount++ // this will be reset to zero when the process has successfully started
+
+	p.proxyLogger.Debugf("<%s> Executing start command: %s, env: %s", p.ID, strings.Join(args, " "), strings.Join(p.config.Env, ", "))
 	err = p.cmd.Start()
 
 	// Set process state to failed