feat: docker build: Add DOCKER_BUILD.md and Dockerfile (#341)

Take the parts from `.github/workflows/test.yml` needed to do full
testing with running `luacheck`, the tests, and coverage.

Use a recent OpenResty 1.29.2.4 Docker image.

Show all steps needed for interactive development in `DOCKER_BUILD.md`.
Also show how to run a single test with `t/01-basic.t` as example.

Author: sriemer

DOCKER_BUILD.md

@@ -0,0 +1,19 @@
+## Build using Docker
+
+Build:
+
+```bash
+docker build -t lua-resty-http-builder:test .
+```
+
+Run:
+
+```bash
+docker run --rm -it -v "$(pwd)":/src -w /src lua-resty-http-builder:test bash
+
+# Check LUA as well as run all tests and coverage:
+luacheck lib && make coverage
+
+# Run a single test file:
+luacheck lib && TEST_FILE=t/01-basic.t make coverage
+```

Dockerfile

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+FROM openresty/openresty:1.29.2.4-alpine-fat
+
+RUN apk add --no-cache curl perl bash wget git perl-dev libarchive-tools nodejs; \
+    ln -s /usr/bin/bsdtar /usr/bin/tar
+
+RUN curl -s -L https://cpanmin.us | perl - App::cpanminus > /bin/cpanm && chmod +x /bin/cpanm
+
+RUN cpanm -q -n Test::Nginx
+
+RUN /usr/local/openresty/luajit/bin/luarocks install luacov && \
+    /usr/local/openresty/luajit/bin/luarocks install lua-resty-openssl && \
+    /usr/local/openresty/luajit/bin/luarocks install luacheck