docker: use .dockerignore to simplify Dockerfile

add Dockerfile to .dockerignore, which achieves the same effect of
quicker iteration, without having to manually add each separate file and
directory to the build image.

.dockerignore

@@ -1,3 +1,4 @@
 bin/*
 tmp/*
 misc/docker/release/*
+Dockerfile

Dockerfile

@@ -18,22 +18,7 @@ WORKDIR /go/src/perkeep.org
 COPY go.mod go.sum ./
 RUN go mod download
 
-# Add each directory separately, so our context doesn't include the
-# Dockerfile itself, to permit quicker iteration with docker's
-# caching.
-ADD .git /go/src/perkeep.org/.git
-add app /go/src/perkeep.org/app
-ADD clients /go/src/perkeep.org/clients
-ADD cmd /go/src/perkeep.org/cmd
-ADD config /go/src/perkeep.org/config
-ADD dev /go/src/perkeep.org/dev
-ADD doc /go/src/perkeep.org/doc
-ADD internal /go/src/perkeep.org/internal
-ADD pkg /go/src/perkeep.org/pkg
-ADD server /go/src/perkeep.org/server
-ADD website /go/src/perkeep.org/website
-ADD make.go /go/src/perkeep.org/make.go
-ADD VERSION /go/src/perkeep.org/VERSION
+COPY . .
 
 RUN go run make.go -v