fix: Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-UBUNTU2004-BASH-581100
- https://snyk.io/vuln/SNYK-UBUNTU2004-NETTLE-1090728
- https://snyk.io/vuln/SNYK-UBUNTU2004-NETTLE-1090728
- https://snyk.io/vuln/SNYK-UBUNTU2004-SYSTEMD-582552
- https://snyk.io/vuln/SNYK-UBUNTU2004-SYSTEMD-582552

Dockerfile

@@ -3,7 +3,7 @@
 # SPDX-License-Identifier: Apache-2.0
 #*********************************************************************/
 
-FROM ubuntu:groovy-20210416 AS rpc-builder
+FROM ubuntu:groovy-20210325 AS rpc-builder
 
 WORKDIR /
 ARG DEBIAN_FRONTEND=noninteractive
@@ -11,7 +11,7 @@ RUN \
   apt-get update -y -qq && \
   apt install -y -qq \
     git cmake build-essential libssl-dev zlib1g-dev \
-    curl unzip zip pkg-config ca-certificates
+    curl unzip zip pkg-config
 RUN git clone https://github.com/open-amt-cloud-toolkit/rpc.git
 WORKDIR /rpc
 RUN mkdir -p build
@@ -23,7 +23,7 @@ WORKDIR /rpc/build
 RUN cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_TOOLCHAIN_FILE=/rpc/vcpkg/scripts/buildsystems/vcpkg.cmake ..
 RUN cmake --build .
 
-FROM ubuntu:groovy-20210416 
+FROM ubuntu:groovy-20210325 
 
 LABEL license='SPDX-License-Identifier: Apache-2.0' \
       copyright='Copyright (c) 2021: Intel'