Added example snippets of the presentation.

Dockerfile

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+FROM r-base:4.1.2
+
+# Install several CRAN packages using package manager.
+RUN apt update \
+    && apt install --assume-yes \
+    r-cran-dplyr=1.0.7-2 \
+    r-bioc-deseq2=1.34.0+dfsg-1 \
+    r-cran-biocmanager=1.30.16+dfsg-3 \
+    r-cran-devtools=2.4.3-1
+
+# Example how to install specific package using R.
+RUN R -e "BiocManager::install('apeglm', dependencies=TRUE, version='3.14')"
+
+# Make a directory in the Docker image and copy scripts to it.
+WORKDIR /analysis/
+COPY ./example/ /analysis/
+
+# Run unit tests.
+CMD R --quiet -e 'testthat::test_dir("test")'

example/example.R

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+# Example function to be tested.
+linear_transformation <- function(x) {
+    return (2*x + 1)
+}

example/test/test_linear_transformation.R

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+source("../example.R", chdir=TRUE)
+library(testthat)
+
+test_that("Real numbers are transformed to odd numbers.", {
+  expect_equal(linear_transformation(3) %% 2, 1)
+  expect_equal(linear_transformation(4) %% 2, 1)
+})
+
+test_that("Positive numbers remain positive.", {
+  expect_true(linear_transformation(3) > 0)
+  expect_true(linear_transformation(10) > 0)
+})

readme.md

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+# Snippets of the `Reproducible and error free scripts` presentation
+Use the `Dockerfile` to build the image, which we name `r_example`:
+
+```Bash
+docker build -t r_example .
+```
+
+As specified in the image's Dockerfile (`CMD` line), by default it will run the unit tests:
+
+```Bash
+docker run r_example
+```