Create docker image for SIPssert. (#24)

* Add docker image + workflow for pushing image.

* Correct context for docker-image.yml.

.github/workflows/docker-image.yml

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+name: Push SIPssert Docker Image in Docker Hub
+
+on:
+  push:
+  workflow_dispatch:
+
+jobs:
+
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: Log in to Docker Hub
+      uses: docker/login-action@v2.1.0
+      with:
+        username: ${{ secrets.DOCKER_USERNAME }}
+        password: ${{ secrets.DOCKER_TOKEN }}
+
+    - name: Set docker-tag variable
+      run:  BRANCH="${GITHUB_REF#refs/heads/}"; echo -e "BRANCH=$BRANCH\nDOCKER_TAG=${BRANCH//main/latest}" >> $GITHUB_ENV
+
+    - name: Build and push Docker image
+      uses: docker/build-push-action@v4
+      with:
+        context: ./docker
+        push: true
+        tags: opensips/sipssert:${{ env.DOCKER_TAG }}

docker/Dockerfile

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+FROM python:3.12-bookworm
+LABEL maintainer="darius.stefan@opensips.org"
+
+RUN apt update && apt install -y tcpdump &&\
+    pip install --upgrade pip &&\
+    pip install pyyaml &&\
+    git clone https://github.com/OpenSIPS/SIPssert.git &&\
+    cd SIPssert &&\
+    python3 setup.py install &&\
+    cd ..
+
+ENTRYPOINT ["sipssert"]
+

docker/README.md

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+## Docker Image for SIPssert
+
+This image can be used to run the SIPssert tool for a test set provided by the user.
+Build the image with the following command:
+
+```bash
+docker build -t opensips/sipssert .
+```
+
+Due to the fact that the SIPssert containers are actually started from the host environment, we need to provide the host's relative path to the scenarios. Since this is unknown in the SIPssert's container environment, we need match the exact host's filesystem layout for the tests directory.
+So, go to your test directory and run the following command:
+
+```bash
+docker run --rm -it -v /var/run/docker.sock:/var/run/docker.sock -v $(pwd):$(pwd) -w $(pwd) opensips/sipssert .
+```