Reworked container build to allow setting instance url at runtime

I patched neko's code to use getenv() to grab instance url at startup and store
it as a global rather than static const.

Dockerfile.build

@@ -0,0 +1,23 @@
+# Container just for building the code and debug
+FROM debian:testing
+
+ENV BUILD_REQ="build-essential libcurl4-gnutls-dev libcjson-dev libpcre3-dev libfcgi-dev git pkgconf libpcre2-dev"
+
+RUN apt update && apt upgrade -y
+RUN apt install -y $BUILD_REQ fcgiwrap
+
+WORKDIR /usr/local/src/treebird
+
+# original source
+COPY treebird/ .
+COPY mastodont-c ./mastodont-c
+COPY treebird/config.def.h ./config.h
+
+# patch or override files
+COPY files/ .
+RUN patch -p1 < instance_env_config.h.patch && patch -p1 < instance_env_main.c.patch
+
+RUN make && make install
+
+# defined only for debugging reasons
+CMD fcgiwrap -c 10 -f -s tcp:0.0.0.0:4008

build_container.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+# builds the 'build' image for compiling and testing
+
+git submodule init
+git submodule update
+
+docker build -t treebird:build -f Dockerfile.build .

files/instance_env_config.h.patch

@@ -0,0 +1,13 @@
+diff --git a/config.def.h b/config.def.h
+index a5eed39..0359cd5 100644
+--- a/config.h
++++ b/config.h
+@@ -32,7 +32,7 @@ static char* const config_canonical_name = "treebird";
+  *
+  * Example: "https://shitposter.club/"
+  */
+-static char* const config_instance_url = "https://my-instance.social/";
++extern char* config_instance_url;
+ 
+ /*
+  * OPTIONAL - Mainly for development (or broken servers)

files/instance_env_main.c.patch

@@ -0,0 +1,29 @@
+diff --git a/src/main.c b/src/main.c
+index fc5cc79..6f3d8bc 100644
+--- a/src/main.c
++++ b/src/main.c
+@@ -20,7 +20,6 @@
+ #include <string.h>
+ #include <mastodont.h>
+ #include <stdlib.h>
+-#include "../config.h"
+ #include "index.h"
+ #include "page_config.h"
+ #include "path.h"
+@@ -41,10 +40,16 @@
+ #include "local_config_set.h"
+ #include "global_cache.h"
+ 
++// global accessed elsewhere via extern def in config.h
++char* config_instance_url = NULL;
++
+ int main(void)
+ {
+     unsigned run_count = 1;
+ 
++    // Env config
++    config_instance_url = getenv("TREEBIRD_CFG_INSTANCE_URL");
++
+     // Global init
+     mastodont_global_curl_init();
+ 

run_container.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+# runs the treebird image as a docker daemon
+
+docker run --rm --name treebird -p 4008:4008 -e TREEBIRD_CFG_INSTANCE_URL="https://sleepy.cafe/" -d treebird:build