Improve the caching behavior of web/Dockerfile.

Previously if anything changed under web/ then the images would get
completely rebuilt. In particular fetching new packages in
`yarn install` can be very slow.

Attempt to minimize file dependencies in earlier stages of the Dockerfile
to maximize the number of layers that can be cached, both in the web
asset stage and the go server stage.

Signed-off-by: Brian Smith <[email protected]>

web/Dockerfile

@@ -4,20 +4,25 @@ RUN curl -o- -L https://yarnpkg.com/install.sh | bash
 ENV GOPATH /go
 ENV PACKAGE github.com/runconduit/conduit/web/app
 ENV PACKAGEDIR $GOPATH/src/$PACKAGE
-COPY web/app $PACKAGEDIR
-WORKDIR $PACKAGEDIR
+
 # node dependencies
-RUN $HOME/.yarn/bin/yarn install --pure-lockfile
+COPY web/app/package.json web/app/yarn.lock $PACKAGEDIR/
+WORKDIR $PACKAGEDIR
+RUN $HOME/.yarn/bin/yarn install --pure-lockfile --frozen-lockfile
+
 # frontend assets
+COPY web/app $PACKAGEDIR
 RUN $HOME/.yarn/bin/yarn webpack
 
 ## compile go server
 FROM gcr.io/runconduit/go-deps:f24620b6 as golang
 ARG CONDUIT_VERSION
 WORKDIR /go/src/github.com/runconduit/conduit
-COPY web web
-COPY controller controller
 COPY pkg pkg
+COPY controller controller
+COPY web/main.go web/
+COPY web/util web/util
+COPY web/srv web/srv
 RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o web/web -ldflags "-X github.com/runconduit/conduit/pkg/version.Version=$CONDUIT_VERSION" ./web
 
 ## package it all up