Correctly set PLATFORM env var in /build.sh.

.travis.yml

@@ -23,7 +23,7 @@ matrix:
     - env: PLATFORM="x86_64"
 
 script:
-  - PLATFORM=$(uname -m) TRAVIS_COMMIT=$TRAVIS_COMMIT ./build.sh
+  - PLATFORM=$PLATFORM TRAVIS_COMMIT=$TRAVIS_COMMIT ./build.sh
 
 
 deploy:

README.rst

@@ -81,6 +81,22 @@ are also encountered in the wild. Other less common or virtually
 unheard of flag combinations (such as ``--with-pydebug`` (``d``) and
 ``--without-pymalloc`` (absence of ``m``)) are not provided.
 
+Building Docker images
+----------------------
+
+Due to the age of CentOS 5, its version of ``wget`` is unable to fetch
+OpenSSL and curl source tarballs. Modern versions of these are needed in
+order to fetch the remaining sources.
+
+To build the Docker images, you will need to fetch the tarballs to
+``docker/sources/`` prior to building. This can be done with the
+provided prefetch script, after which you can proceed with building.
+Please run the following command from the current (root) directory:
+
+```bash
+$ PLATFORM=$(uname -m) TRAVIS_COMMIT=latest ./build.sh
+```
+
 Example
 -------
 An example project which builds 32- and 64-bit wheels for each Python interpreter