Fix Dockerfile

It looks like the working directory of travis only has the actual
mockery binary, and not the repo itself. We probably want to figure out
how to cwd into the directory where the code lives (`$TRAVIS_BUILD_DIR`)
and build inside the container. For now this should work fine.

Dockerfile

@@ -1,11 +1,6 @@
 FROM golang:1.14-alpine as builder
 
-COPY ./ /mockery
-RUN cd /mockery && ls mockery && go install ./...
-
-FROM golang:1.14-alpine
-
-COPY --from=builder /go/bin/mockery /usr/local/bin
+COPY mockery /usr/local/bin
 
 # Explicitly set a writable cache path when running --user=$(id -u):$(id -g)
 # see: https://github.com/golang/go/issues/26280#issuecomment-445294378