Docker Compose environment variable management with .env files

<p>I need to manage environment variables for Docker Compose across multiple environments without committing secrets. I want .env file support with per-environment overrides.</p>

<p>Use .env files with compose variable substitution:</p> <div class="highlight"><pre><span></span><code><span class="c1"># .env (gitignored)</span> <span class="nv">POSTGRES_PASSWORD</span><span class="o">=</span>devpassword <span class="nv">DATABASE_URL</span><span class="o">=</span>postgresql+asyncpg://myapp:devpassword@postgres:5432/myapp <span class="nv">DEBUG</span><span class="o">=</span><span class="nb">true</span> </code></pre></div> <div class="highlight"><pre><span></span><code><span class="c1"># docker-compose.yml</span> <span class="nt">services</span><span class="p">:</span> <span class="w"> </span><span class="nt">api</span><span class="p">:</span> <span class="w"> </span><span class="nt">environment</span><span class="p">:</span> <span class="w"> </span><span class="nt">DATABASE_URL</span><span class="p">:</span><span class="w"> </span><span class="l l-Scalar l-Scalar-Plain">${DATABASE_URL}</span> <span class="w"> </span><span class="nt">DEBUG</span><span class="p">:</span><span class="w"> </span><span class="l l-Scalar l-Scalar-Plain">${DEBUG:-false}</span><span class="w"> </span><span class="c1"># Default if not set</span> <span class="w"> </span><span class="nt">SECRET_KEY</span><span class="p">:</span><span class="w"> </span><span class="l l-Scalar l-Scalar-Plain">${SECRET_KEY:?SECRET_KEY must be set}</span><span class="w"> </span><span class="c1"># Fail if missing</span> <span class="w"> </span><span class="nt">env_file</span><span class="p">:</span> <span class="w"> </span><span class="p p-Indicator">-</span><span class="w"> </span><span class="l l-Scalar l-Scalar-Plain">.env</span><span class="w"> </span><span class="c1"># Load all vars from file</span> </code></pre></div> <div class="highlight"><pre><span></span><code><span class="c1"># Override for staging:</span> docker<span class="w"> </span>compose<span class="w"> </span>--env-file<span class="w"> </span>.env.staging<span class="w"> </span>up <span class="c1"># Extend compose config for prod:</span> docker<span class="w"> </span>compose<span class="w"> </span>-f<span class="w"> </span>docker-compose.yml<span class="w"> </span>-f<span class="w"> </span>docker-compose.prod.yml<span class="w"> </span>up </code></pre></div> <p>Key points: - .env is automatically loaded by Compose -- no --env-file needed for default - ${VAR:-default} sets fallback; ${VAR:?error} fails on missing - env_file passes ALL vars from file; environment overrides specific ones - Commit .env.example (no secrets) as documentation</p>