Only build ARMv5 (fixes #1218)

With this change, the build system only builds one ARM variant - ARMv5.
We call the build architecture simply "arm", as this is what
runtime.GOARCH says.

docker/Dockerfile

@@ -17,9 +17,12 @@ RUN curl -sSL https://golang.org/dl/go${GOLANG_VERSION}.linux-amd64.tar.gz \
         | tar -v -C /usr/local -xz
 
 ENV PATH /usr/local/go/bin:$PATH
-RUN mkdir /go
 ENV GOPATH /go
+ENV GO386 387
+ENV GOARM 5
 ENV PATH /go/bin:$PATH
+
+RUN mkdir /go
 WORKDIR /go
 
 # Use gonative to install native Go for most arch/OS combos
@@ -45,8 +48,6 @@ RUN bash -xec '\
                 for platform in linux/386 freebsd/386 windows/386 linux/arm openbsd/amd64 openbsd/386; do \
                         GOOS=${platform%/*} \
                         GOARCH=${platform##*/} \
-                        GOARM=5 \
-                        GO386=387 \
                         CGO_ENABLED=0 \
                         ./make.bash --no-clean 2>&1; \
                 done \