Add build Dockerfile to build wtfutil from source in Docker and then copy binary to local machine

Dockerfile.build

@@ -0,0 +1,18 @@
+FROM golang:1.13 as build
+
+ARG version=master
+
+RUN git clone https://github.com/wtfutil/wtf.git $GOPATH/src/github.com/wtfutil/wtf && \
+    cd $GOPATH/src/github.com/wtfutil/wtf && \
+    git checkout $version
+
+ENV GOPROXY=https://proxy.golang.org,direct
+ENV GO111MODULE=on
+ENV GOSUMDB=off
+
+WORKDIR $GOPATH/src/github.com/wtfutil/wtf
+
+ENV PATH=$PATH:./bin
+
+RUN make build && \
+    cp bin/wtfutil /usr/local/bin/

README.md

@@ -115,6 +115,19 @@ make install
 make run
 ```
 
+### Installing from Source using Docker
+
+All building is done inside a docker container. You can then copy the binary to
+your local machine.
+
+```bash
+curl -o Dockerfile.build https://raw.githubusercontent.com/wtfutil/wtf/master/Dockerfile.build
+docker build -f Dockerfile.build -t wtfutil --build-args=version=master .
+docker create --name wtf_build wtfutil
+docker cp wtf_build:/usr/local/bin/wtfutil ~/.local/bin
+docker rm wtf_build
+```
+
 **Note:** WTF is _only_ compatible with Go versions **1.12.0** or later (due to the use of Go modules and newer standard library functions). If you would like to use `gccgo` to compile, you _must_ use `gccgo-9` or later which introduces support for Go modules.
 
 ## Running via Docker