ci: add GitLab CI config

.gitlab-ci.yml

+default:
+  tags:
+    - shell
+
+test:
+  script:
+    - docker build -t imagetest:latest .
+    - false
+    - docker run --rm -it imagetest:latest tests/test.sh

tests/expected_output.md5

+6b5a66981bd83329219002897be393a6  test.test-mir.min.1.pileup.tab

tests/test.sh

+#!/usr/bin/env bash
+
+# Tear down test environment
+cleanup () {
+    rc=$?
+    rm "test.test-mir.min.1.pileup.tab"
+    cd $user_dir
+    echo "Exit status: $rc"
+}
+trap cleanup EXIT
+
+# Set up test environment
+set -eo pipefail  # ensures that script exits at first command that exits with non-zero status
+set -u  # ensures that script exits when unset variables are used
+set -x  # facilitates debugging by printing out executed commands
+user_dir=$PWD
+script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 && pwd)"
+cd $script_dir
+
+ascii_alignment_pileup.R \
+  --verbose \
+  --reference="test_files/test.fa.gz" \
+  --annotations="test_files/test.gff" \
+  --output-directory="$PWD" \
+  "test_files/test.bed" \
+  "test_files/test.bam"
+
+# Check md5 sum of output file
+md5sum --check "expected_output.md5"