Dockerfile: support building USB on Alpine, ignore temp files

.dockerignore

+.git
+build/_workspace
+build/_bin

Dockerfile

-FROM alpine:3.3
+FROM alpine:3.5
 
 ADD . /go-ethereum
 RUN \
-  apk add --update git go make gcc musl-dev         && \
-  (cd go-ethereum && make geth)                     && \
-  cp go-ethereum/build/bin/geth /geth               && \
-  apk del git go make gcc musl-dev                  && \
+  apk add --update git go make gcc musl-dev linux-headers && \
+  (cd go-ethereum && make geth)                           && \
+  cp go-ethereum/build/bin/geth /geth                     && \
+  apk del git go make gcc musl-dev linux-headers          && \
   rm -rf /go-ethereum && rm -rf /var/cache/apk/*
 
 EXPOSE 8545

containers/docker/develop-alpine/Dockerfile

 FROM alpine:3.5
 
 RUN \
-  apk add --update go git make gcc musl-dev ca-certificates && \
+  apk add --update go git make gcc musl-dev linux-headers ca-certificates && \
   git clone --depth 1 https://github.com/ethereum/go-ethereum && \
   (cd go-ethereum && make geth) && \
   cp go-ethereum/build/bin/geth /geth && \
-  apk del go git make gcc musl-dev && \
+  apk del go git make gcc musl-dev linux-headers && \
   rm -rf /go-ethereum && rm -rf /var/cache/apk/*
 
 EXPOSE 8545

containers/docker/master-alpine/Dockerfile

 FROM alpine:3.5
 
 RUN \
-  apk add --update go git make gcc musl-dev ca-certificates && \
+  apk add --update go git make gcc musl-dev linux-headers ca-certificates && \
   git clone --depth 1 --branch release/1.5 https://github.com/ethereum/go-ethereum && \
   (cd go-ethereum && make geth) && \
   cp go-ethereum/build/bin/geth /geth && \
-  apk del go git make gcc musl-dev && \
+  apk del go git make gcc musl-dev linux-headers && \
   rm -rf /go-ethereum && rm -rf /var/cache/apk/*
 
 EXPOSE 8545