Split Dockerfile for better build speed, added fixed image tags

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine as curl
+FROM alpine:3.12.4 as curl
 
 WORKDIR /
 
@@ -13,23 +13,40 @@ ARG YQ_BINARY="yq_${OS}_$ARCH"
 RUN wget "https://github.com/mikefarah/yq/releases/download/$YQ_VERSION/$YQ_BINARY" -O /usr/local/bin/yq && \
     chmod +x /usr/local/bin/yq
 
-FROM ubuntu:groovy as fuse-builder
-WORKDIR /build
+FROM curl as img-downloader
+
+ARG IMG_SHA256="cc9bf08794353ef57b400d32cd1065765253166b0a09fba360d927cfbd158088"
+RUN curl -fSL "https://github.com/genuinetools/img/releases/download/v0.5.11/img-linux-amd64" -o "/usr/bin/docker" \
+	&& ( echo "${IMG_SHA256}  /usr/bin/docker" | sha256sum -c - ) \
+	&& chmod a+x "/usr/bin/docker"
+
+
+FROM ubuntu:groovy-20210115 as fuse-downloader
+
 RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get install --no-install-recommends -y \
-        libc6-dev gcc g++ make automake autoconf clang pkgconf libfuse3-dev git \
-        ca-certificates \
+        git ca-certificates \
     && update-ca-certificates && \
     rm -rf /var/lib/apt/lists/*
 
+WORKDIR /build
 RUN git clone https://github.com/containers/fuse-overlayfs.git -b v1.4.0
+
+FROM ubuntu:groovy-20210115 as fuse-builder
+WORKDIR /build
+RUN apt-get update && \
+    DEBIAN_FRONTEND=noninteractive apt-get install --no-install-recommends -y \
+        libc6-dev gcc g++ make automake autoconf clang pkgconf libfuse3-dev \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY --from=fuse-downloader /build /build
 RUN cd fuse-overlayfs && \
     sh autogen.sh && \
     LIBS="-ldl" LDFLAGS="-static" ./configure --prefix /usr && \
     make
 
 
-FROM ubuntu:groovy
+FROM ubuntu:groovy-20210115
 
 RUN apt-get update && \
     apt-get install -y software-properties-common && \
@@ -49,8 +66,6 @@ RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install --no-instal
 
 WORKDIR /app
 
-RUN mknod /dev/fuse -m 0666 c 10 229
-
 COPY dep-bootstrap.sh .
 RUN chmod +x ./dep-bootstrap.sh
 
@@ -59,11 +74,7 @@ USER root
 RUN useradd -u 1000 -s /bin/bash jenkins
 RUN mkdir -p /home/jenkins
 RUN chown 1000:1000 /home/jenkins
-RUN export IMG_SHA256="cc9bf08794353ef57b400d32cd1065765253166b0a09fba360d927cfbd158088" \
-    && curl -fSL "https://github.com/genuinetools/img/releases/download/v0.5.11/img-linux-amd64" -o "/usr/bin/docker" \
-	&& echo "${IMG_SHA256}  /usr/bin/docker" | sha256sum -c - \
-	&& chmod a+x "/usr/bin/docker" \
-    && export IMG_DISABLE_EMBEDDED_RUNC=1 \
+RUN export IMG_DISABLE_EMBEDDED_RUNC=1 \
     && chmod u-s /usr/bin/newuidmap /usr/bin/newgidmap \
     && echo "jenkins:100000:65536" > /etc/subgid \
     && echo "jenkins:100000:65536" > /etc/subuid \
@@ -73,6 +84,7 @@ RUN export IMG_SHA256="cc9bf08794353ef57b400d32cd1065765253166b0a09fba360d927cfb
 
 ENV JENKINS_USER=jenkins
 
+COPY --from=img-downloader --chown=1000:1000 /usr/bin/docker /usr/bin/docker
 COPY --from=yq-downloader --chown=1000:1000 /usr/local/bin/yq /usr/local/bin/yq
 COPY --from=fuse-builder --chown=1000:1000 /build/fuse-overlayfs/fuse-overlayfs /usr/bin/fuse-overlayfs