added instructions to build and run XCA using docker

INSTALL.docker

@@ -0,0 +1,69 @@
+
+This is a short overview of how to build and run XCA using docker.
+
+Install docker
+==============
+
+To install docker, follow the instructions provided at
+	https://docs.docker.com/install/
+
+Build XCA container
+===================
+
+Use the following command to build XCA container:
+
+	docker build --rm --build-arg USER_ID=`id -u` --tag xca .
+
+This will build a container named "xca" using default configuration.
+
+Build options
+-------------
+
+There are some options to customize build. Use dockers "--build-arg"
+option to add options.
+
+CODENAME              : code name of ubuntu version
+                        example: --build-arg CODENAME=bionic
+USER_ID               : id of user "user" inside of container
+                        example: --build-arg USER_ID=`id -u`
+PARALLELMFLAGS        : make flags for parralel build
+                        example: --build-arg PARALLELMFLAGS=-j2
+OPENSSL_VERSION       : version of OpenSSL
+		        example: --build-arg OPENSSL_VERIONS=1.1.1d
+OPENSSL_SHA1          : sha1 checksum of OpenSSL package
+                        example: --build-arg OPENSSL_SHA1=056057782325134b76d1931c48f2c7e6595d7ef4
+OPENSSL_BUILD_PARALLEL: enable/disable parallel build of OpenSSL
+                        example: --build-arg OPENSSL_BUILD_PARALLEL=YES
+                        note: some (older) versions of OpenSSL do not support parallel build
+
+Run XCA
+=======
+
+Once the container is built, run it using the following command:
+
+	docker run --rm -it --user `id -u` --network=host \
+		-e DISPLAY=$DISPLAY -e "QT_X11_NO_MITSHM=1" \
+		xca 
+
+Share local directory
+---------------------
+
+Use docker volumes to share a local directory. This might be useful to store data base files.
+
+	mkdir -p ./some_local_directory
+	docker run --rm -it --user `id -u` --network=host \
+		-e DISPLAY=$DISPLAY -e "QT_X11_NO_MITSHM=1" \
+		-v ./some_local_directory:/backup \
+		xca
+
+This makes ./some_local_directory accessible in the container as /backup.
+
+Run bash
+--------
+
+For debugging purposes container can be started in bash mode:
+
+	docker run --rm -it --user `id -u` --network=host \
+		-e DISPLAY=$DISPLAY -e "QT_X11_NO_MITSHM=1" \
+		--entrypoint dumb-init xca -- bash
+