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ARROW-1991: [Website] Fix Docker documentation build

@kou something is failing here, I'm not sure what's different here vs. Travis CI (not sure if this is what's failing the doc build):

```
  DOC   Building HTML
../arrow-glib-docs.xml:25: warning: failed to load external entity "../xml/gtkdocentities.ent"
  %gtkdocentities;
                  ^
Entity: line 1:
 %gtkdocentities;
                 ^
../arrow-glib-docs.xml:29: parser error : Entity 'package_name' not defined
    <title>&package_name; Reference Manual</title>
                         ^
../arrow-glib-docs.xml:31: parser error : Entity 'package_string' not defined
      for &package_string;.
                          ^
warning: failed to load external entity "../xml/basic-array.xml"
../arrow-glib-docs.xml:43: element include: XInclude error : could not load ../xml/basic-array.xml, and no fallback was found
warning: failed to load external entity "../xml/composite-array.xml"
../arrow-glib-docs.xml:44: element include: XInclude error : could not load ../xml/composite-array.xml, and no fallback was found
../xml/array-builder.xml:25: warning: failed to load external entity "../xml/xml/gtkdocentities.ent"
  %gtkdocentities;
```

Author: Wes McKinney <wes.mckinney@twosigma.com>
Author: Kouhei Sutou <kou@clear-code.com>

Closes #1472 from wesm/fix-gen-apidocs and squashes the following commits:

5b907acd [Wes McKinney] Add explicit instructions for uploading API docs to website
5734a651 [Wes McKinney] Use JDK7 for Java on Ubuntu 16.04
0fcb1e11 [Wes McKinney] Use gcc 4.9 rather than default gcc because of gcc5 ABI issues
dbf8be8e [Kouhei Sutou] Disable auto-reconfigure
b1b5050b [Kouhei Sutou] Fix GLib doc build
8b2d7e43 [Wes McKinney] Fixes for glib doc build
9da9e143 [Wes McKinney] Add BOOST_ROOT