fix(dockerfile): use POSIX-compliant arithmetic for retry backoff

Replace bash-specific exponentiation (5 * 2 ** (ATTEMPT - 1)) with POSIX
sh-compatible case statement for hardcoded backoff values:
- ATTEMPT 1: 5 seconds
- ATTEMPT 2: 10 seconds
- ATTEMPT 3: 20 seconds

This fixes 'arithmetic expression: expecting primary' errors in Alpine
and Debian builds which use /bin/sh instead of bash.

Applies to both:
- Dockerfile.v1: install-php-extensions download and installation
- Dockerfile.v2: s6-overlay download

Author: KingPin

Dockerfile.v1

@@ -31,7 +31,11 @@ RUN for ATTEMPT in 1 2 3; do \
             break; \
         else \
             if [ $ATTEMPT -lt 3 ]; then \
-                SLEEP_TIME=$((5 * 2 ** (ATTEMPT - 1))); \
+                case $ATTEMPT in \
+                    1) SLEEP_TIME=5 ;; \
+                    2) SLEEP_TIME=10 ;; \
+                    3) SLEEP_TIME=20 ;; \
+                esac; \
                 echo "Download attempt $ATTEMPT failed, retrying in ${SLEEP_TIME}s..."; \
                 sleep $SLEEP_TIME; \
                 rm -f /usr/local/bin/install-php-extensions; \
@@ -48,7 +52,11 @@ RUN for ATTEMPT in 1 2 3; do \
             break; \
         else \
             if [ $ATTEMPT -lt 3 ]; then \
-                SLEEP_TIME=$((5 * 2 ** (ATTEMPT - 1))); \
+                case $ATTEMPT in \
+                    1) SLEEP_TIME=5 ;; \
+                    2) SLEEP_TIME=10 ;; \
+                    3) SLEEP_TIME=20 ;; \
+                esac; \
                 echo "Extension installation attempt $ATTEMPT failed, retrying in ${SLEEP_TIME}s..."; \
                 sleep $SLEEP_TIME; \
             else \

Dockerfile.v2

@@ -163,7 +163,11 @@ RUN if [ "$BASEOS" = "trixie" ] || [ "$BASEOS" = "bookworm" ]; then \
             break; \
         else \
             if [ $ATTEMPT -lt 3 ]; then \
-                SLEEP_TIME=$((5 * 2 ** (ATTEMPT - 1))); \
+                case $ATTEMPT in \
+                    1) SLEEP_TIME=5 ;; \
+                    2) SLEEP_TIME=10 ;; \
+                    3) SLEEP_TIME=20 ;; \
+                esac; \
                 echo "Download attempt $ATTEMPT failed, retrying in ${SLEEP_TIME}s..."; \
                 sleep $SLEEP_TIME; \
                 rm -f /tmp/s6-overlay-*.tar.xz; \