Split build and download in Dockerfiles

On systems with low resources the command `go mod download` can fail.
This causes long builds of the docker images. This change adds a new
layer in the docker build splitting the build and download in two
steps.

Fixes #1114

docker/Dockerfile.step-ca

@@ -4,6 +4,7 @@ WORKDIR /src
 COPY . .
 
 RUN apk add --no-cache curl git make
+RUN make V=1 download
 RUN make V=1 bin/step-ca bin/step-awskms-init bin/step-cloudkms-init
 
 

docker/Dockerfile.step-ca.hsm

@@ -5,6 +5,7 @@ COPY . .
 
 RUN apk add --no-cache curl git make
 RUN apk add --no-cache gcc musl-dev pkgconf pcsc-lite-dev
+RUN make V=1 download
 RUN make V=1 GOFLAGS="" build