Fixed install for apt-based images

shell-based if statements do not seem to work within RUN, this
refactoring works without them

wopiserver.Dockerfile

@@ -14,13 +14,8 @@ LABEL maintainer="cernbox-admins@cern.ch" \
 # prerequisites: we explicitly install g++ as it is required by grpcio but missing from its dependencies
 WORKDIR /app
 COPY requirements.txt .
-RUN if command -v apk &> /dev/null; then \
- echo "Using apk"; apk add curl g++; \
- elif command -v apt &> /dev/null; then \
- echo "Using apt"; apt -y install g++; \
- else \
- echo "This distribution does not provide a supported package manager"; false; \
- fi
+RUN command -v apk && apk add curl g++
+RUN command -v apt && apt update && apt -y install curl g++
 RUN pip3 install --upgrade pip setuptools && \
  pip3 install --no-cache-dir --upgrade -r requirements.txt