Better cache optimizing Dockerfile

Dockerfile

@@ -1,39 +1,45 @@
 # docker build . -t cosmwasm/wasmd:latest
 # docker run --rm -it cosmwasm/wasmd:latest /bin/sh
-FROM cosmwasm/go-ext-builder:0.8.2-alpine AS builder
+FROM cosmwasm/go-ext-builder:0.8.2-alpine AS rust-builder
 
 RUN apk add git
 # without this, build with LEDGER_ENABLED=false
 RUN apk add libusb-dev linux-headers
 
 # copy all code into /code
 WORKDIR /code
-COPY . /code
+COPY go.* /code/
 
 # download all deps
-RUN go mod download
-# TODO: how to use this instead of hardcoding GO_COSMWASM
-RUN basename $(ls -d /go/pkg/mod/github.com/\!cosm\!wasm/go-cosmwasm@v*)
-
-ENV GO_COSMWASM="v0.9.1"
+RUN go mod download github.com/CosmWasm/go-cosmwasm
 
 # build go-cosmwasm *.a and install it
-WORKDIR /go/pkg/mod/github.com/\!cosm\!wasm/go-cosmwasm@${GO_COSMWASM}
-RUN cargo build --release --features backtraces --example muslc
-RUN mv /go/pkg/mod/github.com/\!cosm\!wasm/go-cosmwasm@${GO_COSMWASM}/target/release/examples/libmuslc.a /lib/libgo_cosmwasm_muslc.a
-# I got errors from go mod verify (called from make build) if I didn't clean this up
-RUN rm -rf /go/pkg/mod/github.com/\!cosm\!wasm/go-cosmwasm@${GO_COSMWASM}/target
+RUN export GO_WASM_DIR=$(go list -f "{{ .Dir }}" -m github.com/CosmWasm/go-cosmwasm) && \
+    cd ${GO_WASM_DIR} && \
+    cargo build --release --features backtraces --example muslc && \
+    mv ${GO_WASM_DIR}/target/release/examples/libmuslc.a /lib/libgo_cosmwasm_muslc.a
+
+
+# --------------------------------------------------------
+FROM cosmwasm/go-ext-builder:0.8.2-alpine AS go-builder
+
+RUN apk add git
+# without this, build with LEDGER_ENABLED=false
+RUN apk add libusb-dev linux-headers
 
-# build the go wasm binary
 WORKDIR /code
+COPY . /code/
+
+COPY --from=rust-builder /lib/libgo_cosmwasm_muslc.a /lib/libgo_cosmwasm_muslc.a
 
 # force it to use static lib (from above) not standard libgo_cosmwasm.so file
 RUN BUILD_TAGS=muslc make build
 
+# --------------------------------------------------------
 FROM alpine:3.12
 
-COPY --from=builder /code/build/wasmd /usr/bin/wasmd
-COPY --from=builder /code/build/wasmcli /usr/bin/wasmcli
+COPY --from=go-builder /code/build/wasmd /usr/bin/wasmd
+COPY --from=go-builder /code/build/wasmcli /usr/bin/wasmcli
 
 COPY docker/* /opt/
 RUN chmod +x /opt/*.sh