build: trim the size of the Docker container

Prior to this change, the size of the image was about 300MB:
```
; docker images|grep fcuny/golang|grep days
fcuny/golang.fcuny.net             d07add42f21a69e2c057eae8492bbd599dc50082   9fd284c5d8ee   2 days ago       313MB
fcuny/golang.fcuny.net             ca3dd083f8d6642821781ce03829713524322bbe   8104afadfd1f   2 days ago       313MB
```

With this change, we're reducing the image to less than 10MB:
```
fcuny/golang.fcuny.net             19b3dccc1ff31534df3636d4d7d6a28c0ae1e189   c9ae35574f8e   24 seconds ago       9.09MB
```

Since there's storage and bandwidth involved in this process, there's no
point in not going for a smaller image.

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