Enhance docker-build doc for debian

- Older OS release for better portability
- Add hint about musl libc
- Use apt-get over apt cmd to fix api warnings

2 files changed, 18 insertions, 9 deletions

diff --git a/contrib/build-using-docker/Dockerfile b/contrib/build-using-docker/Dockerfile
index 00c1132..47b14f5 100644
--- a/contrib/build-using-docker/Dockerfile
+++ b/contrib/build-using-docker/Dockerfile
@@ -18,7 +18,7 @@ RUN true \
     && $PKGADD $PKGS_TO_ADD \
     && git clone --depth 42 --branch "${GIT_TAG:?}" https://github.com/hiddenalpha/bulk-ln.git . \
     && git config advice.detachedHead false \
-    && if test -n "$GIT_TAG"; then git checkout "$GIT_TAG"; fi \
+    && git checkout "$GIT_TAG" \
     && ./configure $CONFIGURE_OPTS \
     && make clean && make && make install \
     && find . -not -wholename './dist*' -delete \
diff --git a/contrib/build-using-docker/README.md b/contrib/build-using-docker/README.md
index 63b7182..ec571e0 100644
--- a/contrib/build-using-docker/README.md
+++ b/contrib/build-using-docker/README.md
@@ -8,10 +8,18 @@ configure/make/install becomes a nightmare. I found it to be easier to have a
 Dockerfile to build on a totally unrelated machine (but where I have the needed
 privileges) and then just copy-paste the built result over to where I need it.
 
+WARN: Default configuration uses an alpine base image that means build will
+      dynamically link to musl libc and so won't run on most other systems.
+      This is because for a quick-n-dirty walktrough about what this tool does
+      this is ways enough. Consult "Other targets" further down and configure a
+      base image that fits your needs.
 
-## Setup variable to reduce annoying repetitions
+
+## Setup basic stuff to reduce annoyance
 
 ```sh
+mkdir /tmp/some-empty-dir
+cd /tmp/some-empty-dir
 IMG=bulk-ln-showcase:latest
 ```
 
@@ -44,17 +52,18 @@ sudo docker run --rm -ti "${IMG:?}" sh
 ## Other targets
 
 The dockerfile is parameterized and should work for other systems too. For
-example to compile for debian we could use command like below. May set *IMG*
-differently if you need to keep multiple images.
+example to compile for debian we could use example command as below.
+
+To build for other systems it should be enough to adjust below command.
 
 ```sh
 curl -sSL http://git.hiddenalpha.ch/bulk-ln.git/plain/contrib/build-using-docker/Dockerfile | sudo docker build . -f - -t "${IMG:?}" \
-    --build-arg PARENT_IMAGE=debian:buster-20220622-slim \
+    --build-arg PARENT_IMAGE=debian:9-slim \
     --build-arg PKGS_TO_ADD="curl gcc git make libc-dev ca-certificates tar" \
     --build-arg PKGS_TO_DEL="curl gcc git make libc-dev" \
-    --build-arg PKGINIT="apt update" \
-    --build-arg PKGADD="apt install -y --no-install-recommends" \
-    --build-arg PKGDEL="apt purge -y" \
-    --build-arg PKGCLEAN="apt clean"
+    --build-arg PKGINIT="apt-get update" \
+    --build-arg PKGADD="apt-get install -y --no-install-recommends" \
+    --build-arg PKGDEL="apt-get purge -y" \
+    --build-arg PKGCLEAN="apt-get clean"
 ```