Docker needs golang 1.4.2 to build, so we do too

build/Dockerfile

@@ -1,12 +1,21 @@
 FROM ubuntu
 
+RUN apt-get -y update && apt-get -y install --no-install-recommends build-essential git ca-certificates docker.io mercurial libpcap-dev curl make pkg-config gcc bison flex
+
 # When doing a build in a container, "apt-get update" happens twice,
 # which can be a very significant overhead for incremental builds.
 # And it's unnecessary, because if any of the prerequisites do change,
 # the right thing to do is to rebuild the container image.  Rather
 # than suppressing the "apt-get update" commands explicitly, we clear
 # out sources.list so that 'apt-get update' doesn't do anything.
+RUN echo >/etc/apt/sources.list
+
+ENV GO_VERSION 1.4.2
+RUN curl -sSL https://golang.org/dl/go${GO_VERSION}.linux-amd64.tar.gz | tar -C /usr/local -xz
+ENV PATH /usr/local/go/bin:$PATH
+
+# So that sudo does not trample on PATH
+RUN sed -ie '/secure_path/d' /etc/sudoers
 
-RUN sh -c 'apt-get -y update && apt-get -y install --no-install-recommends build-essential git ca-certificates golang docker.io mercurial libpcap-dev curl make pkg-config gcc bison flex && echo >/etc/apt/sources.list'
 COPY build.sh /
 ENTRYPOINT ["sh", "/build.sh"]

build/build.sh

@@ -35,7 +35,7 @@ EOF
     echo "weave:x:$uid:$gid::$WEAVE_SRC:/bin/sh" >>/etc/passwd
     echo "weave:*:::::::" >>/etc/shadow
     echo "weave	ALL=(ALL)	NOPASSWD: ALL" >>/etc/sudoers
-    su weave -c "make -C $WEAVE_SRC build"
+    su weave -c "PATH=$PATH make -C $WEAVE_SRC build"
 else
     # There are arguments to pass to git-clone
     mkdir -p ${WEAVE_SRC%/*}

site/building.md

@@ -31,15 +31,20 @@ $ git clone https://$WEAVE $GOPATH/src/$WEAVE
 $ cd $GOPATH/src/$WEAVE
 ```
 
-Some prerequisites are needed to build weave.  First, install Docker
-if you haven't already, by following the instructions [on the Docker
-site](https://docs.docker.com/installation/ubuntulinux/).  Note that
-we recommend using the Docker-maintained `lxc-docker` package, rather
-than the `docker.io` package which contains a very old version.  Then
-install the other prerequisites for building with:
+Some prerequisites are needed to build weave:
 
+* Install Docker if you haven't already, by following the instructions
+  [on the Docker
+  site](https://docs.docker.com/installation/ubuntulinux/).  Note that
+  we recommend using the Docker-maintained `lxc-docker` package, rather
+  than the `docker.io` package which contains a very old version.  Then
+  install the other prerequisites for building with:
+* Building weave requires Go 1.4.2, and the `golang` package in Ubuntu
+  14.04 LTS is too old.  So you may need to [install the tarball from
+  golang.org](http://golang.org/doc/install).
+* A few other packages are also needed:
 ```bash
-$ sudo apt-get install build-essential git golang mercurial libpcap-dev
+$ sudo apt-get install build-essential git mercurial libpcap-dev
 ```
 
 Then to actually build, simply do: