Build gcc first.

The gcc build step takes by far the longest, and we don't expect
to update it as often as later dependencies. So we trade the
improved security of using a newer-than-system openssl and curl
to download source for the convenience of faster rebuilds by making
it more likely the gcc build will be in docker's cache.

Author: rillian

slaves/dist/Dockerfile

@@ -24,6 +24,13 @@ RUN mkdir /buildslave && chown rustbuild:rustbuild /buildslave
 RUN mkdir /home/rustbuild
 RUN chown rustbuild:rustbuild /home/rustbuild
 
+# Install gcc 4.7 which has C++11 support which is required by LLVM
+#
+# After we're done building we erase the binutils/gcc installs from CentOS to
+# ensure that we always use the ones that we just built.
+COPY dist/build_gcc.sh /build/
+RUN sh build_gcc.sh && rm -rf /build
+
 # We need a build of openssl which supports SNI to download artifacts from
 # static.rust-lang.org. This'll be used to link into libcurl below (and used
 # later as well), so build a copy of OpenSSL with dynamic libraries into our
@@ -40,13 +47,6 @@ RUN sh build_openssl.sh && rm -rf /build
 COPY dist/build_curl.sh /build/
 RUN sh build_curl.sh
 
-# Install gcc 4.7 which has C++11 support which is required by LLVM
-#
-# After we're done building we erase the binutils/gcc installs from CentOS to
-# ensure that we always use the ones that we just built.
-COPY dist/build_gcc.sh /build/
-RUN sh build_gcc.sh && rm -rf /build
-
 # binutils < 2.22 has a bug where the 32-bit executables it generates
 # immediately segfault in Rust, so we need to install our own binutils.
 #