feat: Add override for TeXlive repo

Dockerfile

@@ -97,6 +97,7 @@ ENV OUT_DIR="${out_dir}"
 # Instead of VOLUME, which breaks multi-stage builds:
 RUN mkdir -p "${src_dir}" "${tmp_dir}" "${out_dir}"
 
+ENV TEXLIVE_REPOSITORY=""
 COPY entrypoint.sh /bin/entrypoint
 # Add "aliases" to align `docker run` and `docker exec` usage.
 RUN set -eo noclobber; \

README.md

@@ -58,6 +58,33 @@ Place a file called `Texlivefile`  with a list of required CTAN packages,
 one name per line, in the source directory.
 The container will install all packages on that list before running the work command.
 
+---
+⚠️ Images will stop working once a new version of TeXlive is released with an error like this:
+
+> tlmgr: Local TeX Live (2023) is older than remote repository (2024).
+
+If you need to keep using an older image for a little while, 
+you can override the repository by setting environment variable 
+`TEXLIVE_REPOSITORY` to a value like
+```
+https://ftp.math.utah.edu/pub/tex/historic/systems/texlive/2023/tlnet-final
+```
+This feature has been available since 2024.1;
+see 
+  [reitzig/texlive-docker#18.2022953222](https://github.com/reitzig/texlive-docker/issues/18#issuecomment-2022953222)
+for hints on how to backport it to older images.
+
+⚠️ Note that most CTAN mirrors do not maintain historic versions
+(cf. [tex.SE#460132](https://tex.stackexchange.com/questions/460132/historic-tex-live-distributions-https-sftp-mirror)),
+so keep in mind that widespread use of this workaround _will_ stress those few mirrors that do.
+We strongly recommend upgrading to the latest TeXlive version as soon as possible!
+
+<!-- TODO: provide example-->
+<!-- ℹ️ That said, an alternative is to maintain custom Docker images with historic package versions;
+see [here](TODO) for an example. -->
+
+---
+
 ### Parameters
 
 You can adjust some defaults of the

entrypoint.sh

@@ -34,20 +34,23 @@ Commands:
 
 Environment Variables:
 
-    OUT_DIR         Directory for the relevant output of work commands.
-                    Default: /work/out
-    SRC_DIR         Directory with project sources. Can be read-only.
-                    Default: /work/src
-    TMP_DIR         The working directory for work commands.
-                    Default: /work/tmp
-    BUILDSCRIPT     Script in SRC_DIR that can be run by the work command.
-                    Default: build.sh
-    TEXLIVEFILE     A file in SRC_DIR that contains the TeXlive packages the
-                    project requires, one package name per line.
-                    Default: Texlivefile
-    OUTPUT          Bash glob pattern that defines the relevant output of
-                    work commands.
-                    Default: '*.pdf *.log'
+    OUT_DIR             Directory for the relevant output of work commands.
+                        Default: /work/out
+    SRC_DIR             Directory with project sources. Can be read-only.
+                        Default: /work/src
+    TMP_DIR             The working directory for work commands.
+                        Default: /work/tmp
+    BUILDSCRIPT         Script in SRC_DIR that can be run by the work command.
+                        Default: build.sh
+    TEXLIVE_REPOSITORY  Direct URL to a TeXlive repository;
+                        bypasses use of mirrors.ctan.org and gives access to history versions.
+                        Default: empty (default behaviour of tlmgr)
+    TEXLIVEFILE         A file in SRC_DIR that contains the TeXlive packages the
+                        project requires, one package name per line.
+                        Default: Texlivefile
+    OUTPUT              Bash glob pattern that defines the relevant output of
+                        work commands.
+                        Default: '*.pdf *.log'
 HELP
 )"
 
@@ -94,9 +97,15 @@ case "${command}" in
 
         if [[ -f "${SRC_DIR}/${TEXLIVEFILE}" ]]; then
             if ! sha256sum -c "${hashfile}" > /dev/null 2>&1; then
+                tlrepo=""
+                if [ -n "${TEXLIVE_REPOSITORY}" ]; then
+                    echo "Will use TeXlive repository ${TEXLIVE_REPOSITORY}"
+                    tlrepo="--repository ${TEXLIVE_REPOSITORY}"
+                fi
+
                 echo "Installing dependencies ..."
-                tlmgr update --self
-                xargs tlmgr install < "${SRC_DIR}/${TEXLIVEFILE}"
+                tlmgr update ${tlrepo} --self
+                xargs tlmgr install ${tlrepo} < "${SRC_DIR}/${TEXLIVEFILE}"
                 tlmgr path add
                 sha256sum "${SRC_DIR}/${TEXLIVEFILE}" > "${hashfile}"
             else