Pin R version & update install sequence about openfaas-rstats-templates

[psolymos]

Probably easier to allow changing R image via ARG, here is Ubuntu/plumber example:

• define default ARG that can be overridden at build time: https://docs.openfaas.com/cli/build/#30-pass-custom-build-arguments
• remove dependency on littler, so that rstudio images can be used
• also copy and install deps from DESCRIPTION before moving the /function folder: this improves build caching

ARG WATCHDOG_VERSION="0.8.4"
ARG R_IMAGE="rocker/r-ubuntu:18.04"

#FROM ghcr.io/openfaas/of-watchdog:0.8.4 as watchdog
#FROM rocker/r-ubuntu:18.04
FROM ghcr.io/openfaas/of-watchdog:${WATCHDOG_VERSION} as watchdog
FROM ${R_IMAGE}

COPY --from=watchdog /fwatchdog /usr/bin/fwatchdog
RUN chmod +x /usr/bin/fwatchdog

# Install system requirements for index.R as needed
RUN apt-get update && apt-get install -y \
    --no-install-recommends \
    make \
    git-core \
    libssl-dev \
    libcurl4-gnutls-dev \
    curl \
    libsodium-dev \
    libxml2-dev \
    libicu-dev \
    libstdc++6 \
    && rm -rf /var/lib/apt/lists/*

# Change options as needed in Rprofile.site
COPY Rprofile.site /etc/R
ENV _R_SHLIB_STRIP_=true

# Install basic dependencies for index.R
# some packages seem to require build from source
RUN R -q -e 'install.packages(c("Rcpp", "stringi", "httpuv", "remotes", "plumber"), repos=c(CRAN = "https://cloud.r-project.org"))'
#RUN install2.r -e remotes plumber

COPY install.R /usr/local/bin/

COPY function/DESCRIPTION     .

# Istall SystemRequirements for handler.R from DESCRIPTION
RUN R -q -e 'source("/usr/local/bin/install.R");install$sysreqs()'
RUN echo requirements.txt
RUN apt-get update
RUN xargs -a requirements.txt apt-get install -y --no-install-recommends
RUN rm -rf /var/lib/apt/lists/*
RUN rm requirements.txt

# Install packages (incl. remotes) for handler.R from DESCRIPTION
RUN R -q -e 'remotes::install_deps()'

# Install VersionedPackages for handler.R from DESCRIPTION
RUN R -q -e 'source("/usr/local/bin/install.R");install$versioned()'

RUN rm -f ./DESCRIPTION

# Create a non-root user
RUN addgroup --system app \
    && adduser --system --ingroup app app

WORKDIR /home/app

COPY index.R           .
COPY function          .

# Switch to app user
RUN chown app:app -R /home/app
USER app

ENV fprocess="Rscript index.R"
ENV mode="http"
ENV http_upstream_url="http://127.0.0.1:5000"

HEALTHCHECK --interval=5s CMD [ -e /tmp/.lock ] || exit 1

CMD ["fwatchdog"]