dropboxd: not found

After a system restart, dropbox was no longer working.

Restarting the container (`docker restart dropbox`) gives

```
$ dropbox status
Starting ...
```

And then after a while

```
$ dropbox status
Dropbox is not running
```

I tried removing the container and running `dropbox-start` all over. Log:

```shell
$ docker logs dropbox
Checking for latest Dropbox version...
Latest : 78.4.119
Installed: 11.4.21
Downloading Dropbox v78.4.119...
####### 10.1%
curl: (56) SSL read: error:00000000:lib(0):func(0):reason(0), errno 104

gzip: stdin: unexpected end of file
tar: Unexpected EOF in archive
tar: Unexpected EOF in archive
tar: Error is not recoverable: exiting now
Installing new version...
Dropbox updated to v78.4.119
Starting dropboxd (78.4.119)...
/opt/dropbox/dropboxd: 3: exec: /opt/dropbox/dropbox-lnx.x86_64-78.4.119/dropboxd: not found
Checking for latest Dropbox version...
Latest : 78.4.119
Installed: 78.4.119
Dropbox is up-to-date
Starting dropboxd (78.4.119)...
/opt/dropbox/dropboxd: 3: exec: /opt/dropbox/dropbox-lnx.x86_64-78.4.119/dropboxd: not found
```

Now

```shell
dropbox status
Error response from daemon: Container 714f1e69dc62b947448d8ab7f91c5392f8986d1e9ede094d967178011115e4d6 is restarting, wait until the container is running
```

Start by inspecting the container startup and update path behind `dropbox-start`, then reproduce the failure with `docker restart dropbox` and `docker logs dropbox`. Trace the interrupted download and subsequent missing executable; the issue is done when the container starts Dropbox successfully after a restart and `dropbox status` reports it running.

Written by the indexing model from the issue text.