Stop building packer, use binary provided by their container

Thanks to the following issue hashicorp/packer#8846, and the fact that they’ll be removing abiity to use a simple ‘go get’ (hashicorp/packer#8837 (comment)), it makes no sense to try to build it locally anymore.

Dockerfile

@@ -6,14 +6,12 @@ RUN go get github.com/aelsabbahy/goss/cmd/goss@v${GOSS_VER} && \
     strip ${GOPATH}/bin/* && \
     go clean -cache -modcache
 
-# Build goss, packer, and packer-provisioner-goss with musl
+# Build goss and packer-provisioner-goss with musl
 FROM golang:1.13-alpine3.10 as build_musl_bins
-ARG PACKER_VER=1.5.4
 ARG PACKER_PROVISIONER_GOSS_VER=1.0.0
 ARG GOSS_VER=0.3.10
 ENV GO111MODULE=on
 RUN apk --no-cache --upgrade --virtual=build_environment add binutils && \
-    go get github.com/hashicorp/packer@v${PACKER_VER} && \
     go get github.com/YaleUniversity/packer-provisioner-goss@v${PACKER_PROVISIONER_GOSS_VER} && \
     go get github.com/aelsabbahy/goss/cmd/goss@v${GOSS_VER} && \
     strip $GOPATH/bin/* && \
@@ -29,12 +27,16 @@ LABEL maintainer="Dominik L. Borkowski"
 
 # Get binaries from musl based container
 COPY --from=build_musl_bins \
-    /go/bin/packer /go/bin/goss /go/bin/packer-provisioner-goss /bin/
+    /go/bin/goss /go/bin/packer-provisioner-goss /bin/
 
 # Get binaries from glibc based container
 COPY --from=build_glibc_bins \
     /go/bin/goss /bin/goss-glibc
 
+# Get packer binaries from their official container
+ARG PACKER_VER=1.5.4
+COPY --from=hashicorp/packer:${PACKER_VER} /bin/packer /bin/packer
+
 # Install few essential tools and AWS CLI, then clean up
 RUN apk --no-cache --upgrade --virtual=build_environment add \
         gcc python3-dev musl-dev libffi-dev openssl-dev && \